I have been throwing around the idea of "sleeper agents" for the villain in content. The idea is this: someone dies in a trap—I like to make my traps rather vicious endeavors: a pit trap that falls into a well filled with water and an Owlbear skeleton at the bottom to grapple people. Did I mention the water itself is a large vat of Waters of Lamashtu?—and is offered a raise dead by something. Maybe it is a god watching out for the hero, but it might also be the villain of the dungeon.
The idea is that the BBEG leaves the dungeon after a sleeper agent is created, and instead the final encounter, formerly with the BBEG, is instead an encounter where the sleeper agent backstabs the party.
One concept I had was Billy the Rogue falls into aforementioned trap: a pit trap he failed to notice. He falls 30 feet into a pit of Waters of Lamashtu and is now around 15 feet down in the 20-ft deep pool. The Owlbear skeleton, which has been holding its action to grapple anything that comes within range, activates to grapple the rogue. Each round Billy needs to make a fort save or take int and dex damage, but he also has to try and escape the skeleton before he drowns or is crushed to death. The party, seeing this, decides that Billy the Rogue's survival is a lost cause. They abandon him to his doom, as his CMD decreases due to Dex damage, and his drowning swiftly becomes an inevitability. Just to point this out, his allies could easily lower themselves into the pit, fight the skeleton, rescue their ally and climb to safety no problem: they are just choosing not to.
I hand Billy a piece of paper: a missive from the villain of this dungeon. It details how he could turn on his friends, how the villain will be using him to scry on his allies and send his minions towards them, how he is supposed to aid his friends until the very end or if he knows he could win if he attacked. He can become, for all extents and purposes, a secret agent. His character is dead, but if he wins he gets XP for killing the other members of the party when everyone rerolls. Of course, he can refuse, at which point he stays dead for the piece of content unless brought back by the other players.
However, it is just as likely that a deity might have taken pity on Billy, striken down the owlbear when he died then used raise dead on him, the cost of course coming out of the dungeon treasure pool should he accept: but he doesn't know that nor does the rest of the party.
Do you guys think this could be an interesting mechanic in a piece of content that will harbor the players working together to aid one another?
Sleeper Agents and divine interventioned characters would have Non-detection cast on them until the end of the piece of content, mostly to obfuscate whether or not the character is an ally or spy.
Of course, the dungeon doesn't get easier just because they abandoned someone or let them die, so even if the PCs kill their newly raised ally again just to be sure he wont be a traitor, they are only hurting themselves.
If the character became an NPC, then it may work.
I wouldn't recommend this as a repeat technique, but it can be excellent once.
I will point out that it takes a special player to pull this off. I did something similar in a game years ago. The party had uncovered a powerful artifact and were planning to use it to help solve the problems they faced. The artifact in question had a history of being owned by a succession of highly ambitious and proud military rulers. It was also an artifact forged by a faith that despised what the characters were. After the first battle where the artifact was used, I told the player that the artifact told her that now it was time to defeat the evil creatures she was with. The player's response "okay!".