Inspired by the Alice video game, I've been working on a magical/alchemical substance that I'm tentatively calling murderspray. The subject enters a fugue state, goes on a murderous rampage, and takes on the Half-fiend template for 2d4 hours.
The plot is that a series of murders are occuring in town and the PC's have to chase down the perp. Once they catch a murder red-handed and lock him or her up, they'll think they've got it solved, but, of course, the murders continue. Eventually, the PC's figure out that the alchemist is at the heart of the problem, conducting unethical research, but not before one or more of them suffers a dose of murderspray themselves. Wouldn't it be fun if the PC were a paladin?