1. A caveman has awoken in your home setting, a primordial man. He is spreading primitivism as he goes. In his wake people have fought one another tooth and nail, blood was spilled, lived ruined. He's holed up in the old monastery where his new "tribe" are too cowed to challenge him, afraid of what he might do next.
2. The adventure begins when it's 16 Celsius outside. For every in-game hour that passes the temperature drops by 2. This affects about a ten mile radius around the caveman. He will plunge the region well beyond winter unless he is stopped today.
3. Before the session begins have your players write down 100 words. When they enter the monastery all characters lose their vocabulary except for 1 word, rolled randomly from the list. Wizards get 1+Int bonus words. Nobody can read. Players must communicate using only this word which almost no other character will likely know. Instead of treasure once a concept has been demonstrated enough to be explained, or once a familiar item or idea resurfaces, the DM may allow players to regain another word. Actually speaking of treasure any relics or silver that go missing from the monastery will be chalked up to the mad monks or the caveman, so...
4. Being in the same room as the caveman reduces your Intellect by 1 every ten minutes until you have dumb animal intelligence, say Intellect 2. If you ever reach that you lose even the one word you know. The brothers here are all basically animals now and they will defend their territory on sight. They are young and virile and that's actually weird...
5. When you find the caveman you will fly into an uncontrollable rage and must attack something ever round until the caveman is dead or you are. Keep in mind that the temperature effects and intelligence loss effects will still progress in this period.
6. The caveman has missed a lot of time and he needs to make up for it; yours will do. Every time the caveman touches you or every round he sustains that touch you de-age by a year.
7. Successfully kill or re-Captain America the caveman and you get his extra time, aging 1d100 years and gaining an absolute shitload of XP, a bunch of new languages, and knowledge out of step with your setting, like maybe firearms or trains or lasers or cloning.
8. If the caveman touches the idol mounted in the cloister alcove then the caveman will wither to dust as the Saltpeter Madre takes HIS fluctuating time and awakens herself, like a living statue, from a sleep she entered before the current gods. Do not fuck with her run away just go go away and be somewhere that is not here.