It should be a dark ritual in honor of Lolth to bring more power into the webs she weaves. The result of the players intervening could take 3 different routes.
1. Short-infinite Loop
The adventurers reach the site of a terrible ritual, only too late (example: All You Need is Kill). They are trying to make it through impossible obstacles and regardless of how successful they are or their actions along the way always end up back at the beginning when they die. This encourages a Meta-Game approach to the problem (in character) as the PCs are aware of everything they did before and make it through further and faster each time. Eventually they should be able to undo the cause of the loop by speed running the dungeon.
2. Long Single Loop
The adventures get sent way back in time by the dark ritual they interrupt/disrupt (example: Chrono Trigger). Themes that can be played around with are the absolute isolation of being in a completely different age. They could also be responsible for founding of the underdark and/or the worship of Lolth by their actions in the past. The overall goal should be to get back to their own time since everything they know or care about is there.
3. Short Single Loop
The Adventurers get transported back into their own timeline within a recent period (example: Prince of Persia: Warrior Within) . Their former selves are still active though. This is ripe for the PCs being shown as the cause of some of the problems they faced originally and/or a great way to have the time-traveled versions of PCs work to make things easier for their original selves.