NOTE: HERE BY VANCIL 1418 UNIVERSE PLOT SPOILERS
Time Travel is a concept which appears in all manner of fictional works (but is not, unfortunately, something the Author believes is possible apart from as a kind of consequence of Relative faster-than-light motion, so does not feature heavily into his plot lines). It does, however, exist within a certain fashion in the Vancil 1418 Universe. One of the primary antagonists, Marius Warren, uses a combination of Magic / Thaumaturgy and futuristic technology to travel from an unspecified point in an unspecified future date backwards to the present day reality of the Vancil 1418 Universe, the year 2019. There, he attempts to kill Anneliese Vancil to prevent some horrendous apocalyptic event she will unleash at some point in the future. For more details, see the article on Marius Augustus Warren
How Vancil 1418 Universe Time Travel WorksEdit
As a rather convenient cop-out, nobody is entirely sure. Time travel through purely Magical / Thaumaturgical means is largely the realm of speculation and hoax, at least in modern times. A great many proscribed rites exist by which time travel is proported to be possible. Many of these are incomplete; others, wholly or partially indecipherable. The majority of them, however, are simply fake. A few exist which can be wholly understood but cannot be tested because they would require the sacrifice of several hundred human beings. This is because time travel Magic suffers essentially under the same limitation as teleportation, only much more so.
First, let's discuss teleportation, since the two are related. Magical Teleportation can be done relatively easily in one of two conditions. One, the Practitioner wishes to teleport himself/herself from his or her current position to another one within visible range - i.e. a place he / she can actually see, with his or her own two eyes, unobstructed by any walls or barriers. This is generally called a Displacing Spell. This becomes increasingly complicated as the Practitioner wishes to bring more objects or other people with him / her. It is most easily accomplished from a standing position, relaxed, with plenty of energy and concentration while naked (Thaumaturgically-attuned items such as a Staff not withstanding; these will generally aid in the process rather than hinder it). Still, an accomplished Thaumaturge can use Displacing Spells to gain an upper edge in a fight or for all other such advantages the ability would imply, provided he / she is very well practiced in the execution of the Spell (because it has such a vast margin for error).
The second type of teleportation, much safer but requiring much more energy, is a Recolating - or Relocation - Spell. This can be done just as quickly, but requires more preparation. Essentially, the Relocation Spell is used to move one or more persons / objects from their current position to a pre-designated position within a special Thaumaturgically-attuned circle. Thus, most Practitioners of sufficient skill and means will have, somewhere important or secret for them, a circle of silver inscribed with runes that they can use as a 'fall back point' to teleport to when their lives are in danger. The biggest problem with Relocation Spells is that they can be tracked by even a relatively amateurish Practitioner. A very good Sorcerer or Sorceress would be able to use the Magical conduit created by the Spell (i.e. the 'pathway' between point A and B) to teleport himself / herself to the prepared destination in pursuit, since the energies involved in the Spell would illuminate the path like a Christmas tree as well as provide plenty of power for the chasing Practitioner to duplicate the teleportation.
The same thing is also true of the short-distance 'Displacing' teleportation Spells, but since this is limited to visual range the problem is not so great. However, both types of teleportation spells are extremely uneconomical in their use of energy, and thus require a very good - or very well-equipped - Thaumaturge to be pulled off in most cases. Large amounts of practice with these Spells can reduce the amount of energy they require, but they will never be efficient.
Theoretical Time Travel MagicEdit
Time travel rites or Spells, in theory, could be done the same way as these teleportation spells - but by displacing one's self in time rather than in space. Thus, one could travel to his or her same location in a different time (i.e. alter time but not position), or travel to a prepared destination point in the past. The first method would obviously be more useful, since the second would essentially mean setting up a 'marker' in a particular place and at a particular instance of time and then later traveling back to that place and point. Neither of these methods, however, are regarded as feasible for the vast amounts of Magical energy they require, and no method exists for traveling FORWARD in time. This is regarded as quite impossible, even theoretically.
It is assumed, then, that the futuristic society from which Marius Warren hails has found a way to, either technologically or Magically, convert mundane energy into Thaumaturgic energy (which are not related at all; one is an actual physical thing, the other is not - Thaumaturgic power being merely a force, like gravity). How this is possible is unknown, although it is assumed to be accomplished through some sort of technological means: the Laws of Thaumaturgy, like the laws of Newtonian physics, state that new Magical energy cannot be created; all Magical energy that will ever exist currently exists and has always existed. However, if this could be accomplished, the possibilities are virtually limitless. Of course, this is just pure supposition.
The biggest problem with time travel, apart from the actual near-insurmountable constrains to actually doing it, is the complex nature of time itself. There are an essentially infinite number of possible universes, or 'world lines', all stimming from a single point: the creation of material existence. At that point, the parallel universes begin to diverge along every possible concievable decision point, point of chance, or other variable - forming what are called 'World Lines'. Often times, these lines will merge again later as the points of divergence that separated them were insufficient to alter the continuity of the universe as a whole, generating 'ropes' of closely-related and essentially identical parallel universes. However, even the seemingly-most inconsequential differences can have vast reprocussions: something called the Butterfly effect. Thus, time is neither linear, nor circular - it is more akin to an ever-growing, ever-branching tree all stimming from a single point.
The difficulty, then, in time travel, assuming the purpose is to affect some kind of change, would be in selecting the optimal 'point' and 'world line' in which to make the alteration. In this particular case (that of Marius Warren), the present setting of the Vancil 1418 Universe simply happens to be the ideal point for Warren to insert himself (why he couldn't have simply gone back in time and killed Anneliese as an infant, for example, is unknown, although he himself elaborates on this later in the story, to the best of his limited knowledge). It also worth noting that Warren's mission is a gamble - his masters cannot say for certain exactly what the consequences of his actions will be. For more information on that, also see Marius Warren's relevant entry.