In my world a war happened between a few tribal leaders a really long time ago. Those leaders had nearly god-like magical powers, giving them the ability to modify bodies, matter (like earth, stone, water), and power (heat, light etc.). They also had access to the highest steampunk tech you can think off.
My tribal leaders waged highly destructive wars for centuries, mainly because they couldn't kill each other. Eventually they tried to imprison one another. However, when the damage they caused to the planet became critical, the orginal creator of the world (who was more powerful than all the others) intervened and banished the tribal leaders to other planes.
Most of the creations of the tribal leaders and the remnants of their wars evaporated into a white ash that now covers a great portion of the planet.
What plausible explanations are there for the white ash?