[Gnaskar]

Superhero game: Giant world ending ritual with 666 demons in attendance going on in a city park downtown. The players' weather mage has just gotten divine powers from being infused with angel blood (the last of the players to receive a power upgrade from a "radiation accident"). Quote: "Can I make it rain holy water?" After a thirty minute break I throw away the rest of my plans, and improvise a new and different boss fight.

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Dark Sci-Fi: The players are deep in the bowels of an arcology, on a mission to exorcise a demon in the city's main computer. The computer controls the whole city, including giving orders to the police. The police show up with a SWAT team and order the players to surrender. To my complete surprise, they do. The computer shuts down life support in their cells, and they repeatedly fail to escape. Eventually, the group's mage tries to open a tear in reality to break the cell and loses control of it, effectively ending the campaign with a TPK.

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First game I ever ran. DnD 3.5. No limits on source books to use. One player lists half his items as being from [BoEF]. Takes me a while to find that particular book, since you generally don't expect players to browse through the Book of Erotic Fantasies for utility items. Next campaign, there were a few limits on which source books were available.