Boil's Reaping Inferno

You cast this spell by coating the Archdemon horn with the ash, and then stabbing it into the Dragon Heart while uttering the incantation.

When the spell is cast, all living plants within a 10 mile radius burst into magic flames. These fires are not extinguished by smothering or immersing in water. They will continue to burn and spread until their fuel runs out. Any creature that starts its turn within 5 feet of one of these fires, with the exception of the spell's caster, takes 2d8 fire damage. If this damage reduces a creature to 0 hit points, the creature dies, and its soul is removed from its body as a glowing pale blue light. The soul is pulled into the archdemon horn, where it will remain trapped until the horn is destroyed. A creature whose soul is removed this way cannot be resurrected until the horn is broken.