I have been DMing a Time Travel campaign since quite some time, with characters having the power to travel in time more or less at will (but only as a group), and here is what you should be careful about.

Timeline Alteration

My players are surprisingly responsible and are trying to avoid to change the timeline as much as possible, even when I encourage it. Basically you can allow it, warn them beforehand of the possible consequences, and allow only one in every "era" they are in.

So if they travel to medieval Japan, they might accidentally (or willingly) do something that would shame the image of katanas, and no one would ever dare use a katana again in Japan. If later they accidentally kill the Emperor, I would use the "Well, someone else take his place, and that's who History remembered" trick.

The only kind of alteration you need to be careful about is:

Paradoxos

One way of handling it is the "Time protects itself" trick. Alterations are allowed, but if you do something that would cause a paradox regarding the previous events, Time prevents it. You shoot a bullet at your father before you were born, the bullet disappears. You try to stab him, the knife disappears. You try to strangle him... Well...

It's also a good way of leading the PCs. Sometimes in a fight, they realize that they can't kill their opponents because of Time Self-Preservation, but they don't have any idea of who those opponents are exactly, or why it would cause a paradox if they died. One of the PCs also got hunted by one of his ancestors who was disappointed in him, and tried to murder him repeatedly. That PC was unable to fight back, as it would have caused a paradox. Fun times :)

As said above, you can also use the "Rubber band History" method. They can do changes, but whatever change they make ends up being smoothed over by History. They could kill their grand-father, but their grand-mother actually found someone else, who is who they remembered as their grand-father. Or a character kills himself when he was a baby, only to realize that he was actually a clone from the beginning. You can get more options here, but for a RPG game "Enforced Immutability" and "Rubber Band History" are your best bets.

Como?

This is the easiest one: there is no way to do it in RAW. Done. So, invent what you want! A trip to the Astral plane gone wrong, two "Time Stop" spells interacting.. The parameters you have to consider are:

• Do you want the Time Travel method to be easily reproductible?

• Do you want it to be one way? Two-way (to past and back, or to future and back)? Flexible (from whatever point in time to any other point in time)?

• How accurate do you want it to be? Can characters travel two seconds in the past if they want? Or one day? Or are they limited to specific "nexus" in time?

Conselho Final

DMing Time Travelling is awesome and very rewarding, but you will have to plan a lote e ser muito flexible in your scenarios. Take advantage of it and plan relations and links between different characters and places:

"The witch you just captured has the exact same eyes as the old lady you met yesterday! And you remember the old lady telling you she didn't remember her past and was found injured at around the same age than the witch, years ago..."

...but keep it open and vague, and be ready to have to change it at the very last moment, and to adapt seamlessly:

"So, you killed the witch... Congrats. You go to deliver the human sacrifices, and realize two things: the only one alive is a gravely injured little girl, and she has the exact same eyes as the old lady you met yesterday! You also remember the old lady telling you she didn't remember her past and was found injured at around the same age than the little girl, years ago..."

There are two ways you can handle this.

The Resilient Traveller Theory

The Doctor is a resilient traveller. Everything that has happened in the series has happened to him in his almost 1000 years of life. Doesn't mean it's all happened to anyone else. For instance, the Battle of Canary Wharf had never happened by the time the 11th Doctor met Amy Pond. This is why we are constantly surprised by the aliens, despite them destroying London since the 60s.

To make this work, keep the frame of reference on the players at all times. Play up the difference between what they know and what actually happened. When someone leaves the players frame of reference, there is no guarantee that they will exist next time the players go looking in that time period.

In actual play, playing up that difference would include scenes like:

• PCs have not been born in the new timeline. They have effectively appeared fully formed out of thin air. Imagine Back to the Future if Marty hadn't started fading.
• Big events that the players have been involved in haven't happened. "You remember, Daleks flying all over london. Laser blasts. Giant metal men walking every street in the country? That's odd, it only happened counts on fingers three years ago."
• NPCs are different. That helpful barman who's always willing to listen to your problems is different since you came back. Turns out that his house is being foreclosed in this timeline.
• The time machine doesn't exist anymore. The changes bubbled out and now there's no way to go back and make things like it was. Unless you can rebuild it.
• APOCALYPSE! You screwed up ruim. Time to see what the consequences of your thoughtless actions were.

These should only apply when people revisit a particular time and area; they have more impact when they have a home base they keep coming back to.

Há um Doctor Who: Adventures in Time and Space system put out by Cubicle 7 that may work well for you as a game. It's a light rule system which has a great combat resolution order for the source material. In any round the talkers get to go, then the runners, then the people who want to fight.

Fate, Heal Thyself

This comes from a description of Suserano that I've been given. It sounds like an awesome, yet almost unplayable, game.

First a quick description of the setting. You are avatars of powers that have found themselves one moment before the end of existence. To keep existence going just a little bit longer, these powers figure they have to send you back to fiddle with elements of the past. Imagine Quantum Leap if Beckett was an avatar.

The simplest example is "make sure that this person in New York dies at a particular moment". Easy, right? Just push him in front of a car.

Fate doesn't like this and something happens. The car swerves at the last moment and causes a major pile up, but your target gets up and walks away. Then you get sent back to do it again, but this time, not only do you have to kill this person but there's a major pile up going on at the same time.

To actually achieve your goal, you have to create a major distraction for Fate and on your second or later run, actually do what needs doing.

That's not too bad. But just imagine the situation if you have to help Samuel Coleridge to write all of Kubla Khan without distraction.

You can use this to help protect "key" events from being disturbed. Hitler moves at the last second to avoid a sniper bullet. An extra seed falls to grow a tree in a slightly different place. Somebody has a slightly different surge of electricity in their brain that causes them to think of a different pop song. And so on.

The two most important considerations in my experience are Player Knowledge of the Setting's Timeline, and Change resolution.

Several minor considerations as well exist - Paradoxes, Languages, Mana Levels, Social system changes... but those are secondary.

Player Knowledge of the Timeline

Time Travel in fiction is axiomatically about changing events in the past. The question is, whose past? In a D&D game, players may have only the vaguest sense that the world even has a past, let alone what it is. In a near-future Earth setting, players should have at least some concepts of the timeline. In a Traveller 3rd Imperium game, players can look up key elements easily, but there is little support for actually running back in the early days. (Milieux Zero was badly flawed - and one of the most cited reasons for the ultimate failure of T4, as it was set in the "past" of the OTU... and had the same maps and populations as the 1000 year later main setting from earlier editions.)

When players become unstuck in the timeline, it's very easy to get lost in differences between GM knowledge and player knowledge. Lack of knowledge even figured into the plot lines of some of the non-game time-travel fiction: Voyagers!

If some event in a fantasy timeline is to be changed, it needs to be both iconic and tied to character motivations.

Alterar resolução

If players are mucking about in the timeline, they've got several potential reasons to do so... to undo some error in their own actions, to see what might have been, to escape a timeline that either bores or oppresses them... or because it's simply the conceit of the game, such as games based upon Voyagers!, Quantum Leap or Sliders, where not everyone along is intentionally so.

For those who are looking for changes, figuring out what changes happen because of the PC actions is a strong point of argument for many players. It touches on both "Elastic Time" and "Thread-hopping Traveller" elements, as well as "If I can go, so can they." It also can be the "Nail Mission From Hell," and the more commonly known "Butterfly Effect." (See Ray Bradbury's "A Sound of Thunder" for the origin of the term.)

Elastic Time

No matter what you do, the timeline adapts to minimize your changes. You go kill JFK in 1958 to prevent the Bay of Pigs, RFK becomes president and gets assassinated at Dealy Plaza, having authorized the Bay of Pigs. You kill John Wilks Booth, and a drunken prostitute assaults President Lincoln, fatally wounding him, right after the play. Or just before.

In an Elastic Time game, it really doesn't make a whole lot of difference what the PC's do - when they get home, it's status quo.

Thread-hopping Traveller

The Thread-hopping Traveller is an odd phenomenon - Dr. Who is the archetypical example. The Time Traveller who knows lots, but is never certain what he's going to find after any time jump, because wherever he is, he wasn't, and he might never hit the same strand of time again.

It's a core element of both Sliders and Dr. Who that the Time Traveller is constant, and the universe is not... but the two have different means and understandings. Each jump is, essentially, to a different pocket universe, and so the effects of mucking about are pretty immaterial... unless you have some means of getting back.

Efeito Borboleta

When they get back, nothing is the same. Even the tiniest changes have massive and far reaching consequences. That smoke Joe lit up in 63 AD caused an Italian hurricane in July of 64 AD, preventing the fire of Rome... the heat from it disturbed the chaotic system and that grew over 18 months to cause a strange and unseasonable storm that left rome wet-but-standing. That butterfly Jill stopped from feeding in 152 BC didn't make it, and so there are now almost no ravens in 1960. And because of the lack of ravens, lots of bears and thus people starved, making the western conquest of North America a pushover, forcing the Mormons to flee the US to Siberia, rather than Salt Lake City...

Or, killing one guard in Babylon results in all the jews being put to death, rather than enslaved, in the Babylonian Exile... No Jews, no Christians, No Christians, and almost all post AD500 history is null and void. Islam also is fundamentally changed, if it even comes to exist.

Butterfly Effect is also at the heart of many time travel stories, but it is, most fundamentally, a reason to never return from the past - because anything you do ruins your timeline so badly that it's essentially a different place.

Nail Mission from Hell

A nasty variation on either elastic or butterfly effect timelines is that accomplishing something can be extremely hard, and/or require multiple precise changes with many return trips.

In some such stories, the return isn't a return - merely an info-dump into the past from a future where the protagonist is about to die. A Star Trek: The Next Generation Episode used information-only time looping; Data had to break the time loop by sending messages back to himself. Great as fiction, lame as a game mechanic.

In others, one has to go back, and make a single precise change. Talk to no-one, just whack the needed guy, and get out.

In still others, on might be stuck until some element of the timeline is fixed - the whole premise of Quantum Leap is that he becomes unstuck the moment he fixes the timeline.

In the Nail Mission from Hell, one has not only the issue of player knowledge of the timeline, but also getting the clues on how to succeed into the players' hands.

Paradoxo

Paradox is when you do something that makes it impossible for you to have done that same something. The classic example is the Grandfather Paradox: You go back in time and kill your Grandfather before your parent was born, and you immediately cease to exist, and thus haven't killed your Grandfather. Another is snagging an A-Bomb an blowing up the Manhattan Project while it's still in Chicago.

In Elastic Time, the Grandfather Paradox resolves by simply having your grandfather have been cuckolded... the man you thought was your grandfather never was, but the milkman knocked up your Grandma. The A-bomb gets developed by someone else and due to bad placement, most of the city survives.

In Butterfly Effect, your arrival alone alters things drastically, so you might cease to exist upon return, or even fade out like Marty in Volta para o Futuro.

Even in middle of the road settings, many time travel theorists posit that you can't create a paradox - fate will prevent you from doing anything that would cause a paradox - the A-Bomb fizzles, You merely wound grandpa who miraculously recovers and sires mom.

In Thread-hopping, go ahead and kill Grandpa - it doesn't affect you, because you don't affect your own past, but some other timeline, where you may yet exist, or not, and so in a literal sense, tho' you have the same genes, he's not actually seus grandfather, just the man with the potential to sire another you.

Idiomas

We don't actually know for certain how most pre-modern languages sounded. We can guess, but guessing wrong can be fatal for a time traveller. If I go back even to 1960 and ask for a cell phone, I'll be presumed to be either a monastic or criminal wanting a phone in their cell. In 1700, I'll be considered a madman speaking gibberish. In 1700, a computer is a person rather than a machine. At best, people from 1700 would think my accent strange, even were I to be in an area speaking English - many words have changed a lot in 300 years. The best examples are old rhymes that no longer are.

This is the single-most overlooked time travel stumbling block overlooked in fiction, and it's also frequently overlooked in gaming.

If making accidental time travel, or one-way, it's occasionally fun to have the players have to learn the local langage the hard way.

If Time Travel was planned, it's best to either have some variant on the babelfish or a universal translator as an implant, or to otherwise render it null, because usually, it's not that much fun, especially not every new jump.

Conselho Final

Make certain your players are on board with time travel as a game conceit. Especially since you're looking at D&D. Make certain they know when they're going, why, and what the historic matter is that they are going to change.

And pick a semi-resilient timeline - one that ignores minor changes, but major items can be readily done as simpler nail missions. Have paradox prevention be part of the timeline - anything that would result in paradox simply fails. And when it doesn't fail, it rebounds. Kill enough Grandpas, and turn granny into a roundheel... but you still exist. Kill both granny and grandpa, and mom was adopted anyway.

The question did ask for resources and I have to say that GURPS Time Travel is essential reading here. There are some game stats but most of the book covers the kinds of things all GMs need to know (and there's also a condensed timeline of human history, which might not matter much for D&D).

I would lean towards creating an alternate timeline. The party goes back in time, changes the past, returns to the "present", only to find things they love and care about are not as they remember. The characters themselves are unchanged and know a different history to the people inhabiting the timeline they're in.

Depending on their actions in the past, there could be permanent changes in the way magic works. Magical items may have a side-effect that wasn't present in their own timeline.

Now that they've joined a new timeline, they are dopplegangers of their selves who were born in the new timeline. This prevents autohomicidal paradoxes.

This allows the time travel element to entirely boil down to story telling and does not require any new mechanics, making it suitable for any system.

I have little to offer in terms of actual game mechanics - even if I read a couple of fairly old-school games based on time travel.

I just want to urge you to keep in mind what "time travel" really is at least at the narrative level. It's the chance to redeem some past error, or at least to use hindsight.

A second chance - assassinate Hitler, do not drink too much that fateful night, save your loved one from the fire... or die trying.

Having said this, it works best with just one character (especially because he will often have to meet a future or past version of himself) - so traditional group playing will not work very well, I am afraid. Unless time travel is just a gimmick to go visit some sort of "alternate place".

In any case, if you don't know this already, I suggest you read The Merchant and the Alchemist's Gate by Ted Chiang which is an extremely well written short story about what Time Travel is, and in a fantasy setting, to boot.

Extremely recommended.

With regards to spells to accomplish the feat, I would suggest against making them player available. If they can scoot back-and-forth through time at will, then they will play merry hell with your game planning. Given them specific means of time travel, limited as dictated by your adventure. For example in my game, they had access to a tower that would allow them to travel through time, but was only available to them after certain events had taken place, didn't exist in the time they travelled back to (they had to build it themselves) and then became dormant after they returned.

With regards to memory and/or spell casting, I don't think it's wise playing with these things; if you change character memory as a result of time travel, then you will have endless problems trying to reconcile player and character knowledge, and you'll also have significiant problems coming to terms with exactly who knows what, and when. Keep it simple. Players always know their historiers. NPCs might not always remember then same things the players do.

With regards to paradoxes and similar, it depends on the constraints of your world. In my game it was relatively simple, since they travelled back to a period none of them had much familiarity with, and sufficiently far in the past that any major changes they made could have been smoothed over with the passage of time. Per another answer above, my players were actually very cautious about the time line, and did only what they were supposed to do in the past.

In a game where they travel back to somewhere nearer the "current" time, then you have to be very careful; I think the best solution is to ask (yourself) what it is that they're trying to achieve, and make sure that they have reasons to stick to the plot.

I suggest that, unless they are trying to undo something they did, that you explicitely state that they can't undo anything any PC did. This rule alone gives you a lot of leeway in restricting actions, and most players are willing to accept that they shouldn't be able to change the actions of another player (clearly going back in time to correct a mistake needs to be an exception to this).

Finally, I think you should consider carefully what you will allow them to change, and work within this to allow for them to affect the world they return to. Players love to see that their actions have had an impact, and things as simple as changing the name of a major city in the "present" can be a huge reward for them.

One way it could be:

The characters' existence and past is the result of many observations and reported observations. Not all of those reports have to be true.

Parte 1

While walking home, I cut through an alley where there isn't anybody else and I fall and break my arm. I go to the hospital and say "I fell and now my arm hurts and I think it's broken.

Parte 2

While walking home, I cut through an alley where I don't see anybody else, you step out from the shadows and attack me breaking my arm. You then threaten to kill everyone I hold dear, if I reveal you did this. Believing you will follow through on your threat, when I go to the hospital I say "I fell and now my arm hurts and I think it's broken."

Those two result in the same actions and observations and so it is possible for you to go back in time and be the cause of my broken arm.

The catch is that if your actions don't result in me saying the same thing at the hospital (the next point I am observed), then the universe will start erasing you from existence at the point of the change. If I see you come out of the shadows to hurt me but then you disappear, I am likely to say something about it so your existence at that point must be erased too. The logical conclusion is that if you can't force me to say the right thing at the hospital you will fade from existence at the moment you step into my view.

Since the outcome of your actions isn't known until they occur, you perceive your existence beginning to fade the moment you make a real change (get close enough for me to know you intend to interact with me) and it lasts until the results become fixed. Either your intimidation succeeds at which case you were wholly solid all along, or I'm stubborn and don't go along with it at which point your attack never happened and I did actually trip.

This approach preserves history as the players know it but also gives them some ability to accomplish things in the past if they can figure out how to keep the future aligned. It is essentially a "puzzle" scenario and so your players must enjoy that type of thing. I would also rule that their personal history is untouchable, because they have already observed everything that has thus far happened to them and would already know if they will have intervened in their own timeline.

Better yet,you can just have your players be outsiders. So they can't see their parents or other people that can affect them.So you just lose that problem