Bonfire of insanity

A Bonfire of Insanity is a vile ritual performed during rainstorms by powerful necromancers. They build a pyre of human bones and burn it as the rain pours. As the smoke from the fire rises to the clouds, the rain itself is tainted for a mile around the ritual site. Corpses buried in even the most sacrosanct earth rise as undead creatures and any humanoids that the rain touches are infected with a disease that will transform them into a ghoul by the next day unless a heal spell is cast upon them.