So your lovable, incompetent PCs are doomed. They are cornered and facing the might of the entire orcish horde at once. Or, they’ve managed to somehow combine all three encounters in your dungeon into an single skull-crushing reaping. Or, the party wild card insisted in attacking the transdimensional death lord who they were supposed to negotiate with. For whatever reason, you decide that you don’t want to make all of your friends tear up their character sheets. Because it is Not Yet Their Time. Here’s what you do.
Start writing down everything the characters say in-game.
The party appears—but it is the party from the future. They’ve gained a few more magic swords, and the wizard has glowing eyes now for some reason. It’s pretty obvious that they’ve gained a few levels, too.
Anyway, the party-from-the-future proceeds to win the fight for the beleaguered PCs. It’s a tough fight, even with their additional levels and powerful gear, but in the end, they triumph over certain doom.
“Don’t ask us questions. You’ll only invite a paradox,” says the wizard-from-the-future.
“What if we took their gold?” asks the thief-from-the-future, twirling two adamantine daggers as he regards the party-from-the-present.
“Were you paying attention to anything that was said before we came here?” scolds the wizard-from-the-future, looking at the greedy thief incredulously.
As the thief-from-the-future loots all the corpses, he turns to the thief-from-the-present and says, "I'm just going to hold on to this for a while, okay?"
“Hey, Fighter,” says the fighter-from-the-future, who is now missing an arm. “When you get to the red lever, don’t pull it, okay?”
“Aagh! Just forget he said that,” says the wizard, and the party-from-the-future disappears after taking all of the party’s loot.
The party is left confused and hurt, but alive.
Later in the Campaign:
If you know what sort of equipment the party is going to pick up ahead of time, be sure that the party-from-the-future has it visibly equipped. Not only does it give the party a sense of “heehee, we are destined to loot this tomb”, but you can also use it to inject some foreshadowing into the game.
For example, if the wizard-from-the-future was wielding a staff shaped like a three-headed snake, and the party later sees that the vizier has a staff that looks like a three-headed snake, they might assume that the vizier is a bad guy that they will kill and loot later. Or they will do a great favor for the vizier and he will give them a staff. Or the vizier will die in a merfolk attack. Either way, they’ll pay more attention to the vizier later on.
Sooner or later, too, the party will be faced with the Red Lever. It will seem like a good idea at the time to pull it (except for the extra-temporal knowledge the party has). If the fighter pulls the lever, he will lose an arm and preserve the time continuum. If he doesn’t pull the lever, he creates a paradox, and is instantly turned into a Time Zombie. Bear in mind, that the Red Lever can be anything that seems like a good idea but has a bad consequence. Such as: freeing the imprisoned dragon; drinking from the Cup of Roil; reading from the Book of Black Portals.
This takes place as soon as the party is high enough level to overcome whatever dire situation you used Gambit #4 to originally save them from.
It is early one morning. The party has just overcome some huge obstacle the night before. The party is tired, their reserves are depleted, and they are looking forward to a well-earned rest.
Time seems to stop. The party watches an enormous tortoise—six feet tall—crawl up to the party and deposit a letter in the wizard’s pocket. Then, slowly, the tortoise walks out of the room. The party is frozen in time (indeed, the whole world is frozen in time) and can do nothing else except watch the tortoise take his sweet time delivering the letter.
As soon as normal time resumes, the party is all excited. The bard drops the eggs he was making into the fire. Even the fighter rouses himself halfway to yell at the party to keep it down.
The wizard reads the letter from Tovavel, the Tortoise God of Time.
“Remember when our future selves saved us from Hell’s Armada?” asks the wizard.
Everyone nods. It’s tough to forget when you were saved by your future selves.
“We need to go do that. If we don’t, we create a paradox, and we’ll all turn into horrible time zombies.” The wizard explains.
“Should we tell our past selves about the zombie-thing?” asks the bard.
“No, tell them nothing,” says the wizard.
“I said keep it down!” grumbles the fighter.
“Well, let’s get to the chapel. We need to heal Fighter. How long do we have?” asks the bard.
“About 90 seconds, starting from when this letter was delivered.”
When the party goes back, be sure to recreate the fight down the smallest detail. If you wrote down what was said the first time, the party-from-the-past will say those same lines during the combat. Be sure to give them all the loot that they stole from themselves earlier. Remember, that if they try to give the loot to their past selves, they will create a paradox, and turn into horrible shuddering zombies.
Of course, none of this explains why Tovavel, the Lord of Time, went out of his way to save a bunch of unwashed adventurers. Perhaps he just want to preseve a certain timeline. Or maybe he's about to ask for a favor.
Disclaimer #1: I don’t like heavy-handed DM caveats. I believe that if the PCs make their bed, they should have to sleep in it. I also don’t like fudging the dice—but that doesn’t mean that I haven’t done it.
Disclaimer #2: I still maintain that time-travel is a silly idea. Stable time loops are no match for a lively chaos theory butterfly.