An endless hallway, a key that opens 10 doors each day, go too far down the hallway and you travel back in time. Once in the past, you can only move forward.
The first session ended with the following:
Your lives would have been so much simpler if that had been the last time you had seen the key or hallway. History would have turned out so differently if the key had never come into your possession, or if you had been able to resist its allure. Perhaps this time will be different.
The PCs obtained the key in session 8. They entered the same day (of course). Then 1200 years earlier....
http://www.enworld.org/forum/general-rpg-discussion/277635-ten-doors-pick-one-tell-me-why.html