Eresh

Eresh is a creation of the Aszur, just not intentionally. Before the rule of the Eternal King, the Aszur were misguided; self-indulgent and obsessed with excess.Hedonism was at the very core of society and culture. Through this; the Aszur birthed its greatest foe, Eresh into this world. It feeds on one's desires; it delves deep into the darkest reaches of one's mind, festers, and influences one's actions to serve the will of Eresh. All Aszur aspire for their souls to return to Elylerrium, but Eresh feeds on the Aszur souls. Once it corrupts, that soul is forever trapped within, and sustains the power of Eresh. For the souls trapped within, it is eternal damnation and torture.
The Eternal King and the grand forces of the aszur were able to defeat Eresh and save the elves from certain damnation as a race. However, while Eresh is imprisoned in the Mystborne Isle, they still have somehow had their foul influence seep outside.