DMing time travel in PF/3.5? Anyone done this?

[Von Zinzer]

So I've been planning a PF one-shot for some friends in which the BBEG is hell-bent on achieving immortality through space-time manipulation (and threatening the stability of the continuum as a result). My chief inspirations are Homestuck's 'Midnight Crew' intermission, for the mansion setting, sinister tone, and big crew of baddies; Day of the Tentacle, for the mansion again and the idea of sharing items across time frames; and various Star Trek treatments of time travel, in particular causality looping like in 5x18 "Cause and Effect". Basically I want to have the party (level 8) assaulting a mansion stronghold to find the macguffin, get the BBEG, and rescue their kidnapped comrade. I hope to use the "bigger on the inside" trick for the mansion by providing [dimension] doorways as portals to other sections of the mansion that are not necessarily contiguously located in space. In addition to that, I want to have at least three other portals:

1) to a past version of the mansion
2) to a future version of the mansion
3) to a pocket universe/prison cell/safe haven where the party can rest up

So naturally, this is going to be rife with problems. Most obviously is the potential for universe-breaking causality problems and complete dismantling of the game world. I think I've got a handle on that by fluffing the mansion itself, or at least parts of it, as immutable or temporally stable or some such. I believe I can trust my players not to meta-game too hard or to deliberately break things even if I leave holes.

(I also want to implement a madness system drawing from Heroes of Horror so that each time a party member passes through a portal they run a chance of going a bit nuts. I think that'll work fine.)

Has anyone tried this kind of thing before? What pitfalls am I going to encounter? Help me out and I'll be both eternally grateful and shall gladly report back on development/how it goes.

[Telonius]

Spoilering in case any of my players are around ...

I'm going to be doing some time travel in my "Bell, Book, and Candle" campaign - basically, the baddies are conducting a major ritual that's attempting to focus Sonic energy to create a permanent interplanar gate to Nessus. By PC intervention, the ritual is to be averted, but not before several major powers (read: all of the Lords of the Nine) escape, albeit in restricted form. In the course of the disruption, the PCs are flung a thousand years in the future, to deal with the aftermath of the Archfiends' dominion (Samurai Jack-style), and possibly reverse the course of events.

In general, you should probably decide exactly how your causality is going to work. Single timeline (paradox is impossible), multiple timeline (paradox is possible), or ball of timey-wimey stuff (paradox happens when the DM says it happens). The players are probably going to want clarification. Whether you give it to them directly out of character, or demonstrate it by events in the game, it's going to be an important piece of the puzzle.

I'd make it extra-clear what the effects of things like bags of holding or portable holes would be in this pocket dimension. Having the players blow up the universe right after purchasing items would probably be no fun.

For official WOTC time travel stuff: this series of articles (http://www.wizards.com/default.asp?x=dnd/pg/20030402x) might be of interest to you. Quarut from the Fiend Folio is an Inevitable charged with maintaining the space-time continuum. (He's pictured next to the TARDIS-colored Varakhut, who protects deities).