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Background & motivation

On this question about weather prediction, user dsollen commented that someone could potentially use Druidcraft to predict the future, using the following steps:

  • Make a plan with a chance of success
  • Plan, in addition, to use Control weather to create an otherwise unlikely weather phenomena if the plan succeeds.
  • Use Druidcraft to predict the weather within the 24 hours prior to the event, effectively determining whether or not the plan succeeds based on the presence of your chosen weather pattern in Druidcraft's prediction

Whether or not the above is actually possible is out-of-scope for this question (That has its own question here). It just serves to show that rules interactions in 5e could be used to create time-travel paradoxes (in this instance, because reliable information is being sent backwards through time, and future events might change based on that information).

The Question

Are there any rules, for any edition of D&D, which give guidance on how to manage time travel paradoxes? In the absence of such rules (or, where relevant, in addition to them), are there any canonical pieces of lore which inform us of how time-travel paradoxes might resolve or be resolved?

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    \$\begingroup\$ Are you sure Druidcraft foretells the weather by supernatural knowledge? I would think the "nature spirits" just have a highly tuned sense of the atmosphere and how it works -- in other words, they predict the weather the way people have always done, only better. If a wildfire breaks out that causes its own weather pattern, or some dummy casts "Control Weather", there's no way they could have predicted that. \$\endgroup\$ Sep 17, 2021 at 5:26
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    \$\begingroup\$ It doesn't give any actual rules, but Out of the Abyss p153 has "Echoes", which are ghostlike echoes of people who have visited the library of Gravenhollow in the past or in the future. Elminster specifically is mentioned as coming from the future, and the guidance given is pretty much a barnum statement ("...many great heroes were lost to the demons..."). \$\endgroup\$
    – Rycochet
    Sep 17, 2021 at 14:17

2 Answers 2

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Try AD&D 2e's obscure product Chronomancer.

This product (which is long out-of-print but purchasable in PDF form) includes rules for time manipulation, paradoxes, etc. It includes spells explicitly designed to cause paradoxes, such as the 3rd-level spell minor paradox, the more powerful 6th-level spell paradox, and the immensely powerful 9th-level spell major paradox. Chapter 6 is all about the consequences of using such magic and "guidelines" for DMs "to solve any problems that might crop up." (See p. 64). Chapter 3 also lays out ideas for time-traveling adventures and some of the player-facing difficulties that result from altering history.

As for lore, the only setting-specific lore on time travel of which I am aware comes from Forgotten Realms, and is detailed in this Q&A. Chronomancer, meanwhile, contains setting-agnostic lore involving "Temporal Prime," posited as "a pseudo-reality that permeates every plane of existence among the alternate realities," which the authors suggest was previously mis-described in other D&D products as a "Demi-Plane of Time." (See p. 4).

Finally, for the sake of completeness, in 3e/3.5e there were a few less-helpful, less-detailed, and in some cases less-official references to time-related gameplay:

  • The 3e Manual of the Planes presents a "variant plane" called the "Temporal Energy Plane." Its description mentions time loops, but not time travel or paradoxes per se. (See p. 208-210.)
  • An article on Wizards of the Coast's website entitled "Perilous Gateways: Portals in Time" sets out a teleport through time spell that provides a mechanism for time travel. The article goes on to warn the reader about paradoxes and offers the hand-wavey suggestion that "it is simplest to use the rule that changes in time are minor and somehow time smooths them out."
  • Dragon Magazine #350 presented an article on "Chronomancy," which included the statement that "Currently, no spell, artifact, or creature in D&D can time travel over a span of years." The time-related mechanics presented in the article do not permit time travel per se. The article suggests that time travel might be "the purview of deities of travel and time[.]" (See p. 76-79.)
  • Dragon Magazine #359 included a write-up on a monster called a "time dragon," the Great Wyrm form of which has a Time Apotheosis ability that permits it to travel backward and forward through time more or less arbitrarily. The material does not engage the possibility of paradoxes.
  • There were a couple of psionic powers, such as the 9th-level power time regression (Expanded Psionics Handbook p. 138) and the more limited 3rd-level power forced dream (Magic of Eberron p. 104) that allowed the user to move backward in time by very small increments, which theoretically might be capable of creating paradoxes. None of that material explicitly addressed how such paradoxes might be handled.
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    \$\begingroup\$ The Dragonlance campaign setting and attendant stories included some time travel. There was also a dungeon published in Dungeon Magazine called "The Anvil of Time," which involved time travel to three different eras as its gimmick. \$\endgroup\$
    – GMJoe
    Sep 15, 2021 at 22:28
  • \$\begingroup\$ I think the Dark Sun campaign setting books for 2e have a time travel psionic power as well, either in Dragon Kings or The Will and The Way, I'll try to check. Still, great answer, so complete. \$\endgroup\$ Sep 15, 2021 at 22:44
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    \$\begingroup\$ It's worth pointing out that the Netheril Empire 2e box set contains explicit discussions on travelling back to the time of the Netheril Empire, and the restrictions that have been placed on that journey (some of which are to explicitly try and prevent paradoxes from occurring) \$\endgroup\$
    – illustro
    Sep 16, 2021 at 22:09
  • \$\begingroup\$ @illustro Because of the way the question is worded ("are there rules, or else if no rules, what about lore?") I am loath to delve further into Realms-specific lore beyond my one brief mention of it. There are rules, so that is where I focused. I feel that the Netheril stuff would be better discussed in the Realms Q&A I linked to, which is already quite detailed (and even mentions Netheril, though not the box set per se). \$\endgroup\$
    – screamline
    Sep 16, 2021 at 23:06
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    \$\begingroup\$ @gatherer818 There was one Dark Sun region published that had the potential for PCs to retroactively wipe it from existence, depending on actions taken while traveling back in time. \$\endgroup\$ Sep 17, 2021 at 14:15
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The wish spell allows (very limited) alterations to the past in most editions

One of the the options for Wish in D&D 5e is (emphasis added):

You undo a single recent event by forcing a reroll of any roll made within the last round (including your last turn). Reality reshapes itself to accommodate the new result. For example, a wish spell could undo an opponent's successful save, a foe's critical hit, or a friend's failed save. You can force the reroll to be made with advantage or disadvantage, and you can choose whether to use the reroll or the original roll.

The first boldface phrase indicates that the spell literally changes the past by altering a "recent event". It's not quite time travel, but it still has the potential to create a paradox. The 2nd boldface phrase is all the direction given to the DM for resolving any such paradoxes that arise from altering the past event. (The 3.5e version of wish contains the exact same phrases.) Obviously this guidance leaves things almost entirely to the DM's discretion. However, if you find it useful, you could apply this guidance to time paradoxes caused by other means as well, though obviously doing so becomes substantially more difficult if the time span involved is longer than one round.

The OD&D limited wish spell (described here) also mentions altering the past, but as far as I can tell doesn't provide any detail or guidance on how the DM should implement it.

So unfortunately, while wish technically presents an official mechanic for altering the past, it provides minimal guidance for how the DM should actually adjudicate this mechanic.

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    \$\begingroup\$ Maybe the answer doesn’t need this, but would you mind entertaining me for a moment and conceiving of a paradox that you could create by changing something that happened in the last six seconds? \$\endgroup\$ Sep 15, 2021 at 23:00
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    \$\begingroup\$ @ThomasMarkov main paradox is that if a Wish was a response to a friend'd death due to a failed save, and reversed it, then Wish wasn't cast because death didn't occur. But of course without that wish he is dead so caster have casts that wish. And so on ad nausea. \$\endgroup\$
    – Mołot
    Sep 15, 2021 at 23:44
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    \$\begingroup\$ @Mołot you don't need as significant an event as a death. The paradox is intrinsic to the causal chain of the spell. Event happens -> Cast Wish -> Event did not happen -> Why cast Wish? The paradoxical loop is always present with this use of Wish, no matter if the event is a death or a stubbed toe or a spilled drink. \$\endgroup\$ Sep 16, 2021 at 0:26
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    \$\begingroup\$ @ThomasMarkov Obviously you would never actually do this, but in theory you could wish to force yourself to reroll a saving throw that you already passed against a fatal effect, causing you to fail the save and die before you had the opportunity to cast wish. (For a slightly more plausible example, imagine two opposing mages attempting to do this to each other.) \$\endgroup\$ Sep 16, 2021 at 2:19
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    \$\begingroup\$ You have all done a good job of showing why some guidance here would be helpful :P \$\endgroup\$ Sep 16, 2021 at 13:08

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