As the name suggested, a bonfire was needed to cast this spell. The spell could only be cast while it was raining. On casting, the smoke of the bonfire mixed with the rain clouds. This caused a process that filled the rain clouds with vile energy that then rained down using the rain as its medium within 1 mile (1,6 km).
On hitting the ground, corpses, even those that were already skeletal, were raised as ghouls. Humanoids that were hit by the rain contracted a disease. This disease turned the humanoid into a ghoul that was starved and hungry for flesh after twenty-four hours. The caster was not safe from it. Curing this disease was only possible with magic that was as at least as powerful as a heal spell.
Bonfire of insanity required somatic, verbal, and material components. The material component was a bonfire of a diameter of at least 10 ft (3 m) made of bones of at least twenty humanoids that were consumed in the process of the casting and therefore not in danger of rising as ghouls. The spell took ten minutes to cast.