A comet, a rogue ball of methane ice and dust is passing close enough to be seen clearly with the naked eye. Astrologers claim that the end is come, that the sign of the elder and evil gods is riding high in the sky, repent or forever be cast into damnation.
Riding on the coattails of the uproar, banditry has skyrocketed, as commoners lay down their tools and farming equipment, cast aside their bonds to the land and their lords to go afield. In a short time, many cult cells form around charismatic apocalyptic preachers. Some preach ritual suicide to gain spiritual blessings from the the omen of fire, others promote other religious extremes.
Bonfires roar as questionable literature is burned, hermits and magi are hunted down as harbingers of absolute doom, and chaos ensues. The church is rocked as heretics become as common as locusts, preaching their own interpretation of the holy scriptures.
Along for the Ride - The PCs are swept away with the tide of madness, pillaging and looting as these are their last days.
Against the Tide - The PCs have to stand and defend their stronghold against the tide of madness. This could be a personal holding that has been targeted by Bonfire of the vanities fanatics, a chuch against agnostic berzerkers, or the walls of a palace against rioting commoners. The theme would be survival against massive odds.
The Final Days - Perhaps the comet is an omen and the gates of the nether realms are cast open, and the characters have to strive to survive the apocalypse.
We over-reacted - after a few days of mayhem and carnage, the omen of fire passes and vanishes from the sky. The end doesnt come, and after alot of hard, often violent work, everything is brought back into line. The PCs are rewarded for bravery, or called to account for their actions.