Destroyer Breath

Wholsale destruction of civilization was not enough for The Destroyer . In its wake pools of noxius, invisible, purple and pink gas would pile up in dips and caverns. Breathing the gas created horrifying mutations. Flesh could melt from bone, entire persons would turn to goo, one may lose their mind and sprout wings from their back.
They say the gas was a intelligent magical response from the destroyer to the underground hideaways many created when it came near. Carving the earth above, it would then let loose the gas to claim those who hid. Theories abound that this breath alone is the source of over half of the world's now monster-filled underground dungeons.