The Life Bomb

A spell that kills someone by turning them into an explosive. When they die, they erupt and release all the energy that they'd have expended in the next year or the rest of their life or whatever. This turns into a blood-red fire, expanding to something like 100' per class level/hit die, dealing damage based on their highest stat (probably a d6 for their highest stat's bonus, so a guy with a Wis of 16 would do 3d6). Minimum 1d6, of course.

Some of these are 'contagious': if you are killed by an eruption, you create one yourself. This can create a chain reaction sufficient quite easily to empty out a village or a city.

The PCs could come across a testing ground for one of these life bombs, a village that's completely emptied out, pocked with hundreds of scorched craters where people once stood. It would be a great super-villain plot to try and find a villain and prevent them from unleashing the same horrible device on a more densely populated city.

Alternatively, instead of affecting normal folks perhaps it affects only magic users. It could deal damage based on the highest-level spell the MU can cast. This fire would be more silver and green, perhaps, less blood-red.