Time travel (plus prophecies and other devices dealing with time) can be an excellent addition to a roleplaying campaign. Used well, it can tie an adventure together and give a greater sense of scope, making the players feel that they are involved in a plot that spans centuries. However, done badly, it can either be a straight-jacket (if the GM insists that everything must happen exactly as he planned) or pointless (if nothing works out even remotely as planned)… a balance must be struck where the players have enough room to maneuver, but that the future predictions (or whatever device is used) are solid enough to still have purpose.
There are three main variants that I am going to discuss; prophecies, time travel to the future and time travel to the past. I’m sure there are other ways to mess around with time in a game, but most will fall under these broad umbrellas.
For the purposes of this discussion, these could be literal prophecies, clairvoyant visions, predictions by super-smart machines, whatever… anything where the GM gives the players some information that says, “this is what is going to happen.”
This is a dangerous device to use recklessly as it’s a prime opportunity for the GM to look foolish. If your players are anything like mine, attempting to predict what they’re going to do next is an exercise in futility, so you should never make prophetic announcements regarding player actions. However, there are a couple of ways that it can be used in relative safety; either by predicting something that is already happening or by giving instructions to make something happen.
The first of these is very straightforward… finding an ancient prophecy that informs the players about something that happened last week is a very standard way to kick off an adventure. (“Hey, the prophecy says that great evil will arise when red messenger completes its journey. Didn’t that red comet finally disappear last month? We should check this out.”) The players have no opportunity to mess with the sequence of events, as they have already happened
The second variant is also fairly safe, as any instructions remain valid, even if the players don’t follow them. “Those who seek refuge will find it in the serpent’s mouth” is the sort of thing that either provides a useful clue at the right moment (to escape a trap, for example) or remains a mystery if the players don’t encounter the situation it was intended for. In either case, it holds up as true, whether the players follow the advice or not.
Time travel (future):
In many ways travelling to the future in a game is the safest of all time related shenannigans because there is nothing wrong with players changing the future. In fact, it’s a very standard narrative device to show the players an undesirable future and let them actively work to improve it.
Caution still needs to be exercised to ensure that no weird loops are set up, but ultimately anything that doesn’t pan out exactly as the future predicted can be waved away as having changed due to the players’ knowledge of future events.
Time travel (past):
Time travel into the past is probably the most risky proposition of all from a GMing point of view. There are three main approaches that can be taken to changing the past…
1) You can’t, it’s already happened – This provides the most stability to a campaign, but can also require a hefty amount of railroading if your players get creative.
2) You can change the little stuff, but the timeline is self-correcting (kill Hitler and someone suspiciously similar will take his place) – This is handy in games like Feng Shui that involve a lot of time travel, but can also feel like a bit of a cop out.
3) Changing the past is perfectly possible – Most satisfactory from a player point of view, but it can be a nightmare to GM.
None of these approaches is perfect and I’ve never included time travel to the past in my campaigns for that very reason. However, I think that option three is probably the most workable in one of two situations. If the time travel is only back by a day or two, this is fairly easily dealt with; it’s effectively a do-over of recent events, so there are no major repercussions that need to be figured out. Alternatively, if the time travel is sufficiently far into the past (hundreds of years) and you don’t give the players chance to do anything truly catastrophic, you can lean towards option two without needing any mystical timeline inertia… the course of history is different, but the major events (industrial revolution and such like) would probably still happen, so the present remains recognisable. Travelling far enough back also discourages players from trying to alter the timeline in specific ways, as the ripples are too great to predict.
Overall, I’d encourage any DM who wants to include time travel in their campaign to give it a try; just don’t plan anything too smart, as the old (if slightly re-interpreted) maxim is true… no plot survives first contact with the players.