This trap is system independent. It works best when stumbled on in a campaign where the players are visiting past events magically, or are time traveling regularly.
Setup: The party comes across a hallway with windows on either side that look out into duplicate hallways. One of which has duplicates of themselves laying dead in the center. As they enter the room, doors slams shut behind and in front of them and some theme-relevant damage over time (water rising, poison gas, magical lightning, so on) fills the room and starts killing the PCs.
When the party dies, which should take about 10 rounds of damage, the party walks into the hallway next to the now dead party and the cycle begins again.
Solution: The party cannot save themselves, they can only save themselves in the next iteration of the trap. There is a plate in the center of the hallway that dramatically increases the damage of the trap for each PC that stands on it. If all of the PCs lie dead on this plate, the next iteration of the trap will only slam the door behind them, letting them escape into the next section of the dungeon/building.
William Brall
William is the owner and operator of The Bronze Coin, a roleplaying guild located in Reisterstown, Maryland. He's also a Web Developer at Zenimax Online Studios where he built the UI for the patcher/downloader for Elder Scrolls Online.