Lorewise, what is the "Dark Ash" ?

I'm making my own campaign with Draw Steel and am finding some lore implications for the different powers of the game. In my vision :

Essence is pure energy coming from the elements and mainly the sun (wich is quintessence).

Piety is a Saint having interest in you.

Drama comes from the fact that reality was created like a piece of art by the 7 first gods.

But I have no cool idea for the black ash. What is it ? Does it come from an alchemical conception from the Caelian empire ? Did someone find a cool explanation for it's origin ?

Thank you dear community! Always a blast to share ideas with you.