At first Warlock appears to be a simple film:  the titular character is swept from 1691 to perhaps 1989 (the year the film was released), pursued by a hunter intent on bringing him to justice; the warlock is killed, the hunter Giles Redferne says farewell to the heroine Kassandra with a K, and is returned to his own time.  There is, however, a problem:  Redferne uncovered his own crypt.

Perhaps it was not a problem; perhaps no one was in the crypt, but it was simply a tomb created as an intended resting place for a man who vanished and was presumed dead.  But no, someone was buried there.

Perhaps it was unimportant--something that happened in a later adjusted history which is of no consequence other than to confront Redferne with his own mortality.  Unfortunately, the crypt is critical in the history that matters.  It is a serious complication in the film, warranting examination.

The Warlock, who is not named even in the credits, is awaiting his elaborate execution, secured in a tower cell in Boston.  Redferne, the witch hunter who put him there, has just visited, and there is some banter about Satan not having freed the Warlock--yet.  Then as Redferne is leaving, there is an abrupt atmospheric disturbance, and the hunter races back in time to see his prisoner vanishing.  He leaps into the indoor whirlwind after him, and he, too, vanishes.

The Warlock crashes, unconscious, through a window into the California home of Chas, where Kassandra rents a room; they stash him for the night, and after Kassandra leaves for work in the morning the Warlock kills Chas.  He then finds, uses, and murders a "spiritualist" through whom he contacts the devil for instructions.  He has now made a bargain, to find the three parts of Satan's Bible and reunite them, and so become the true son of Satan who can uncreate the universe with a word.  His quest returns him to Chas' house, where the first part is hidden in what was once a communion table, but not before Kassandra has has met Redferne and delivered him to the police.  The warlock curses Kassandra such that she would age rapidly, so she frees Redferne and joins him on the chase, at least until she can undo the curse.

When the Warlock gets the second part of the book, Redferne deduces the goal, and Kassandra rushes them to Boston to learn the location of the third part.  It was buried with Redferne, in 1719; they recover it from the coffin just as the Warlock arrives.  There is a battle in the cemetery, Kassandra delivering the fatal blow, and then barely has Redferne thanked her than he vanishes once more, leaving a fresh inscription on his tomb.

Obviously there is a lot of magic here; there are also a few difficulties that magic will not overcome.  We will begin to unravel them next.

The obvious problem is that Redferne finds his own grave.  As we noted, it will not do to say that someone else was buried in it; the body must be his.  Yet if we follow replacement theory, once Redferne leaves the past and arrives in the future, there must exist a history of the universe in which he vanished, never to be seen again until the late twentieth century.  After all, if we say that he left 1691 and arrived in 1989, we must mean that something happened between 1691 and 1989 that is the history of the world he skipped, and he was not present for it.  Were he to die in 1989, never to return to 1691, there could be no tomb bearing his name or containing his body.  Therefore until he returns a history exists in which that tomb does not.

Redferne is not the first time traveler to see his own grave.  When Scrooge does so in A Christmas Carol, it is best understood as a vision of what might be, not a trip to the future; but Marek did so in Timeline.  That is similar to this, but also quite distinct from it in important ways.  For Marek, who traveled from the future and died in the past, it is simple enough to suppose an original history it which there was no sarcophagus, and then Marek made his trip, died in the past, and his future younger self then discovers the crypt he will in his turn leave in the past.

Thus at first we might think it is not so big a matter this time.  In the original history Redferne does not find his own grave. because he has not yet returned to die in the past.  He goes to the cemetery to get the book, and he is not buried there.  The problem, though, is that the book is buried with him.

It appears that Redferne himself had suggested that this evil grimoire be broken into three parts and moved to three separate protected places.  Someone decided that his grave in the consecrated cemetery was ideal for one of these.  As long as any part of the book remains in a consecrated place, no witch can recover it.

This perhaps poorly-communicated detail explains much of the movie's premise.  At some point, perhaps a century or two ago, the old communion table containing the first part was taken out of service and became an ordinary antique, eventually landing in Chas' home.  The second part got separated from whatever mission church contained it and landed in the attic of a Mennonite farmer perhaps a century ago.  The third part remained safely in Redferne's tomb.  However, Boston redevelopment has recently moved his grave from the original cemetery to an adjacent non-consecrated location, which meant all three parts were now not on holy ground; the devil now can recover it, and so brought his Warlock to the present to retrieve the three parts.

What this suggests to us is that the demon Zamiel reached back from the future to grab the Warlock, accidently picking up Redferne in the process.  That gives us an original history in which both humans died in the past, the book was buried with Redferne, and then Zamiel altered history by snatching them from the past.  It also creates our first genuine problem with the story from a time travel perspective.

We uncovered the first serious problem in the fact that Redferne finds the third part of the evil book buried in his own grave.  We are working on the assumption that the demon Zamiel watched for all three parts of his book to be moved from holy ground, and then reached back from that point in the future to draw his warlock there, picking up Redferne in the process.  That, though, becomes problematic.  Let us trace those events as they must have happened.

At some point, the Christians took possession of the Grimoire.  (Why no one used it to unmake the world prior to that is a mystery beyond the scope of our interest, but it does seem a bit ridiculous that the Satanists had it for however long they did and did not so use it, and as soon as our Warlock has it in his possession his first notion is to destroy everything.)  Having gained possession of this most deadly weapon of their enemy, they agree to Redferne's suggestion that they break it in three pieces and separate them, placing each on holy ground so that no witch nor warlock can retrieve any one.

Since Zamiel, in this view, is waiting to see when his book is available and then rescuing the warlock, it must be that in the original history the warlock was executed in 1691; Redferne's taunts that the devil had not saved him hit home, and the response that much can happen in a day turned to be an empty retort.  In 1692 the book is split into three parts, one placed in a communion table in the church, one given to a minister heading west, and one buried with Redferne.  This is a temporal continuity problem, because the date which appears on Redferne's tomb is 1719, but the note that says the book was buried in that tomb was written in 1692.  Perhaps, though, Redferne's grave was prepared in advance; perhaps the book was stored within it in anticipation of his burial, which took longer than anticipated.  It seems unlikely that a 1692 note would have identified the location of the book as being in a 1719 grave, but it's not impossible.

Eventually Redferne dies.  He might have died in 1692, but that does not really solve our problem--the book was found in a tomb dated 1719, based on directions in a note dated 1692, so the problem exists in the timeline we observe.  But we'll allow that the book and the body both land in the crypt.

Zamiel waits patiently, watching as the communion table is sold to a used furniture store, the minister's old books get left in the attic of his family farmhouse, and finally Boston Redevelopment moves Redferne's grave to a new location on unconsecrated ground.  Then he opens a time portal and draws the warlock out of 1691 and into 1989.  Redferne sees him departing, and catches his coattails to travel with him, something like the Enterprise does to the Borg in Star Trek:  First Contact.

However, Redferne has now vanished from the universe in 1691.  He will not be buried in 1719; nor will there be any plans in 1692 to bury him in that cemetery.  Zamiel has not merely brought two people to the future; he has removed them from the past, such that history has changed.  That means the book is not in Redferne's tomb, given that not yet having returned to the past he cannot have a grave on this first excursion to the future.  In this history Redferne never reappears in 1691, so the decision concerning where to hide the book is made in 1692 without reference to his influence, and it is not in his grave.

However, Zamiel chose this moment because Redferne's grave was moved to its new location.  Since that is no longer relevant, Zamiel will not bring the Warlock and Redferne there.  The question, then, is where they go instead.  That is, the book likely will be uncovered eventually, and when it is Zamiel will be waiting, and will snatch the warlock from his 1691 cell, accidentally bringing Redferne with him to that time.  There are in this instance four possibilities; we will consider them next.

Zamiel brought the warlock to 1989 because the third part of the book, buried in Redferne's tomb, had at that moment been removed from consecrated ground and could be retrieved by a witch.  We also observed that in accidentally bringing Redferne with him, he removed the book from the tomb and placed it somewhere else, and that this meant the book would not have become available at that moment and Zamiel would not have opened the portal.  We now examine the four possible outcomes of that.

It is possible that the book was placed somewhere that would remain consecrated to the end of time.  If it were placed in a tomb beneath a church, the ground of the burial chamber would remain consecrated.  The church itself might fall into disuse, but whatever was done with it in the future, the burial site beneath it would remain consecrated.  We might suppose the church would be demolished; but the burial chamber would remain beneath it as civilization built atop it.  Perhaps someday it might be desecrated, when Boston itself is little more than an archaeological dig site; but perhaps the world will end before that happens.  However, in that case there would never be a version of history in which the book was in Redferne's tomb, and the story could never begin.

Perhaps it is placed on holy ground elsewhere and also meets the eventual fate of moving to non-consecrated ground, but sometime after 1989.  That, though, creates a new problem.  In the original history, we have Zamiel waiting for the pages to become available, and then snatching the warlock when they do; this moves the pages elsewhere, with the result that in 1989 Zamiel is still awaiting their emergence, and he fails to grab the wizard.  When he fails to do so, though, he restores the original history in which the pages are in Redferne's tomb.  We have an infinity loop, in which if the trip is made it will not be made, and if it is not made it will be made.

The next logical possibility is that the book is placed somewhere where it will be available before 1989.  This is considerably more complicated.  What does not matter, though, is whether it is still last available.  If it is, the warlock will be sent thence; if it is not, he will be sent to the time of one of the other parts.  Thus whether it emerges in 1720 or 1920 is irrelevant; we will assume the latter.

In 1989, Redferne's grave containing the pages is moved, and Zamiel snatches Redferne from the past.  This creates a new history in which the pages are not there.  In that history, the pages become available in 1920, and Zamiel, who has been waiting for the book to appear, immediately takes the warlock--and Redferne--to 1920.  The battle which we saw fought in 1989 would then be fought in 1920, absent the assistance of Kassandra.  If the Warlock wins, he uses the book to unmake the world, and there is no 1989, and Redferne is never taken there--but he is still snatched from 1692, because the trip to 1920 replaces the trip to 1989, and now since 1989 will never arrive it does not matter that that trip is never made.  However, it also means that the story we see will never happen, because the moment Zamiel pulled his travelers to 1989, he undid 1989, replacing it with a trip to 1920.  So for the movie to happen, the Warlock cannot succeed in 1920.

The alternative is that Redferne succeeds in 1920; the warlock is killed.  We may reasonably assume that the witchhunter is immediately returned to his own time.  That means we have a trip to the past, and history is altered.  In this new altered history, Redferne dies and is buried, probably in that tomb in 1719.  The book will almost certainly be buried with him--we know he had some influence in the matter, and since he knows now that the book will not be safe wherever they had placed it, he might well argue that it needs to be placed in the cemetery as a safer place.  The problem is that when we hit 1920, Zamiel will not pull the warlock forward to obtain pages that will remain safe until 1989, and therefore Redferne will not be there to make the return trip to 1691, and we again fall into an infinity loop.  Even though Zamfiel originally pulled Redferne to 1989, Redferne never returned from that time, and now he never returns from 1920 either, so he fails to return, time is destroyed, and we never reach the events of the movie.

There remains one more possibility.

We have come to the rather awkward problem that the third part of the book cannot be in Redferne's grave the first time Redferne travels to the future, and thus Zamiel will take the warlock to the time that it is released from whatever other holy ground secured it.  Yet if it was kept secure longer, we have an infinity loop when Zamiel does not grab Redferne in 1989, and if it was released sooner we have an infinity loop when he subsequently does not take Redferne to the earlier time.

That leaves one possible solution:  the change in the location of the pages does not change the time they were moved from holy ground.  Of all the improbable coincidences, it must be that absent Redferne's grave, the pages became available at the same moment they would have had they been in his tomb.  The best hope is that it is still somewhere in the cemetery.

That's not actually so difficult as might at first appear.  After all, even given the size of Boston in 1719, the number of consecrated places which could be expected to remain so in future centuries was fairly limited, and since one part was put in a church and the second sent with a westbound churchplanter, a cemetery makes good sense.  It provides consecrated ground and, in the minds of the seventeenth century, a near certainty that it will remain undisturbed for millennia, less likely to be moved or repurposed than a church.  The same seventeenth century colonists who buried Redferne will have buried many others in that same piece of ground.  There will even be someone in the same plot which we know as Redferne's--the notion of there being an empty grave because he was never buried is foolish, as the specific space for his tomb would have been used for another in his absence.  The grave would have been similarly marked; the story would be much the same, but that Redferne did not find his own grave.  Thus when Boston Redevelopment starts moving graves, they rather methodically move the ones from the same general area of the graveyard--and thus from the same approximate time--at about the same time.  If Zamiel is not timing this to the nearest second, but doing it perhaps at midnight or some similar time, then the fact that that batch of graves were all moved on the same date is sufficient to keep the time trip at the same time.

This also solves another problem easily overlooked.  It is doubtful whether Redferne could have defeated the warlock without Kassandra, particularly in that last battle when she thinks to fill her insulin syringes with salt water.  Whether that was from the brackish outlet of the Charles or from the harbor is not important, but what matters is that the cemetery had to be that close to the ocean that she was soaked in it and so recognized the weapons at hand.  Had Redferne fought the warlock at another time or in another place, he might well have lost.  Having the pages in the same part of the same cemetery not only preserves the timing of the time travel, it also preserves most of the important details of the battle.

Thus we have an original history in which the pages are buried with Redferne, and then in 1989 Zamiel accidentally unmakes Redferne's grave, so in the new history they are buried in a different sepulchre in the same part of the same cemetery, and in 1989 Redferne and Kassandra defeat the warlock and Redferne returns to his own time.  He is then again buried in the third history, so this time in 1989 he finds his own grave, and we see the events of the movie.

There might be ways to simplify this.

It is complicated, because in order for the demon Zamiel to know when his book will become available he must know the future, and to know the future, there must be a future, and for there to be a future there must be a history that leads to it.  Yet when he then pulls the warlock and Redferne to that future, he alters that history, possibly in critical ways.  Is there a way to avoid this?

We might suppose that Zamiel does not, in the strict sense, know the future.  Rather, he has incredibly developed precognitive skills, as we see in Next and Minority Report.  This, though, is somewhat unsatisfying.  We would be suggesting that this potent supernatural being can predict to within a few days the desecration of holy ground three centuries in the future, yet cannot anticipate either that he would accidentally transport the witchhunter or that the warlock would be killed trying to join the three parts.  Unless we want to suppose that Zamiel brought his warlock to the future so that the man could fail, we must assume a strange combination of extremely accurate prognostication of an event that could have happened any time over several centuries with a failure to see the more predictable consequences of his actions.

There is, however, another way to bring the warlock and Redferne to the correct point in the future without knowing the future in advance.  It requires us to rethink the time travel in its forward motion.  Remembering that Rip Van Winkle managed to leap ahead past the American Revolutionary War simply by drinking a draft that caused him to sleep through it, and that when Buck Rogers (and other lesser-known science fiction characters) slept through several centuries he did not age at all, we could suppose a magical counterpart:  Zamiel did not reach back from 1989 to 1691, but rather in 1691 he dropped the travelers into a timeless pocket dimension of some sort where they existed as history advanced without them, then in 1989 released them into the flow of time afresh.  There thus would have been no time travel of the sort we imagine at all; the travelers were simply displaced because they slept through the centuries and did not age during that time.  The advantage here is that there is no original history in which these men died and were buried only to be snatched from the past by someone in the future to create a new history in which they were absent.  The original history in this case is one in which they were removed from time for centuries and reappeared later, taking a vacation not in Boston but away from Boston.

The downside of this solution, though, is that it makes the already confusing return of Redferne to 1691 completely incomprehensible.  If we were to assume that Zamiel opened some kind of time portal from the future to bring the warlock to that moment, and that Redferne was anchored to that time by the presence of the warlock, then the obliteration of the warlock could logically mean he was no longer tethered to the twentieth century and would bounce back to his own time (more immediately than he does in the film, but then, we wanted to see him say goodbye).  If, though, we solve our first problem by removing our travelers from time in the original history, we eliminate any logical reason for Redferne to be returned to the past when the job is completed.

Also, even if we eliminate an anomaly by taking the absence of the travelers as the original history, we still have the anomaly created when Redferne is reinserted into the past.

There is no clue as to the mechanism for Redferne's return to the past; that it makes little sense on any explanation of his trip to the future is a problem we cannot resolve.  The best we can say is that it managed to save us from a schmaltzy Hollywood happily-ever-after ending (highly improbable with Redferne so seriously out of his own time), although whether a disappointing girl-abandoned-by-fate ending is all that much better is a tough call.

The complication that we can address, though, is that Redferne's return to the past gives us a new history, and it might matter.

The primary reason that Redferne's return to the past might matter is that when he was in the future he learned both what was originally done with the three parts of the book in 1692 (the year after his trip) and where they were by 1989.  He knows that the church communion table will be sold eventually to a private owner, that the part given to the mission church will wind up in a farmer's attic, and that the part buried in the cemetery will be moved by secular leaders in a secular future with no conception of the importance of holy ground.  In short, he knows that those hiding places will prove unsafe ultimately.

We must then ask whether he has any influence over where those parts of the book might go, and whether he would exercise it.

To the first question, it apparently is so that he had some influence.  His comment in 1989 suggests that splitting the book and securing it in separate locations was his suggestion, and since the note giving their locations was written in 1692 he was alive at the time those decisions were made.  Further, the elders in Boston knew that something supernatural had happened, that he followed the warlock wherever the warlock was taken, and he returned, so although his story is remarkable their own evidence gives it credibility, and if he claimed to have seen some of the future they would not doubt his word.

The problem that emerges, though, is what we might call the moving pages.  Let us suppose that in the original history the third set of pages is in the coffin of someone in that cemetery otherwise unknown to us.  After Redferne returns, it is clear that the pages were moved to his coffin, but still were moved at the same time.  Did he have influence in that?  Did he suggest perhaps that William's coffin would not be safe, so they put it in his instead?  It might well have been thus.  If so, however, would not the fact that the pages became available in 1989 cause him to suggest putting them elsewhere--perhaps in William's tomb, or in another cemetery?  And thus history could not stabilize, because every time Redferne returned from the future, he, ignorant that he had suggested the given location last time, would suggest a different location.

However, what we do know is that the pages were not in Redferne's (non-existent) tomb in the original history, and were in the second history; we know, too, that in the second history the first two sets of pages were available sooner than the cemetery set.  Finally, we know that when at the end of the second history Redferne travels to 1692, time continues; and thus we must conclude that he changed nothing between the second and third histories.  Perhaps he thought it would be dangerous to alter the future, and so kept his knowledge to himself.  Perhaps he decided that it all worked right in the end and ought not be changed.  Perhaps the elders were afraid of him and did not wish someone who had traveled elsewhere by demon interference to know where the book would be placed.  For whatever reason, it is possible for history to continue beyond Redferne's departure only if that is history's final form, and it is possible for that to be history's final form if he does not attempt to change it upon his return, and thus that must be what happened.

Thus although there are some unanswered questions about the time travel itself (why Redferne returns), the film works as a time travel story.

To this point we have been analyzing Warlock under replacement theory--the only theory of time travel at present which allows a time traveler to travel to his own past and change history.  We have concluded that it works, given that Redferne makes no effort to change where the pages of the grimoire are hidden, and given whether or not Redferne is buried in the past, the third set of pages were always buried in the same cemetery.  Yet some will say that this story works as well under fixed time theory, in which the past cannot be changed; they are correct--but again, with some caveats.

The obvious advantage of fixed time here is that we do not have to worry about what happened when Redferne was not in that coffin; no such history exists.  The history of the world includes that Zamiel moved Redferne and the warlock from 1691 to 1989, then Redferne returned from 1989 and reported the defeat of the warlock in the future, then he died and was buried with the pages of the book, then after the other sets of pages became available the grave was moved in 1989 to unconsecrated ground, the warlock and Redferne arrived and battled, and the warlock was defeated.  Redferne vanishes from the future (because he has already arrived in the past) and Kassandra deals with the disposition of the book.  There is a sort of reverse predestination paradox, in that Redferne finds the grave he is destined to fill, but it's a minor problem as fixed time stories go.

The problem is that one of the key characters lacks motivation.  That is, if Zamiel knows that the final part of book will be available in 1989, he also knows that Redferne will be be taken with the warlock and will defeat him--if not because he can see what happens in 1989, then because he can see what happens in 1691 when Redferne returns.  The issue becomes why Zamiel, in full knowledge that his plan will not only fail but get one of his most promising servants horribly killed, would ever initiate the plan.

Here is where we find our ultimate problem with fixed time:  motivation becomes irrelevant.  Zamiel thinks he is choosing of his own free will to do what he does, but he cannot help doing what he does, even when he knows that it is a mistake.  He might know that all of this will result in failure, but that he is fated to do it.  But then, we are confronted with the problem that he is apparently fated to do it enthusiastically, doomed to act as if there is hope when he must already know it does not exist.  Ultimately, in fixed time theory, it is not only the past that is immutable; it is also the future.  We are not only fated to do what we have already done in the past; we are fated to do what we have not yet done in the future.  We are so fated even if we know incontrovertibly that our acts are both futile and meaningless, and worse, that they will bring our own downfalls.  Free will is illusory; no matter what you know about the future, you will do what you know will cause that future, however undesirable it appears.

In that sense, fixed time theory absolves everyone of his actions.  Criminals cannot help committing crimes, and law enforcement cannot help punishing them, and it has nothing to do with justice but is simply inevitable.  Saints deserve no admiration and sinners no condemnation because they cannot be other than they are; and we cannot help admiring and condemning because we are fated to do so, even when we know that it is meaningless.  Redferne could not in justice destroy the warlock nor oppose him, because the warlock was fated to do all that he did; similarly, Redferne was fated to do as he did, and so was Kassandra, but we cannot admire them for doing so, nor say that either side was "good" or "right" nor the other "evil" or "wrong".  Everyone did what he was doomed to do.

Fixed time theory thus makes sense of some time travel stories, but it creates more problems than it solves.