Salethar

Salethar is the son of Asmodeus (Overlord of Hell) and Sarastra (Fey Queen of Night and Magic). He himself is a Tiefling warlock whose patrons are his parents. Born between worlds, he was never accepted fully as a member of either. While he's the heir to both Hell and the Fey Wild, he spends his entire life fighting for that right. This is largely due to the fact that his parents were both sealed away as part of a divine prophecy. A prophecy only he would dare fulfill. Only when the three main realms merge (The Material Realm, the Fey Wild, and Hell) can his parents be brought back. And so he recruits other like-minded Warlocks (See Allak and Zhalsuth). He uses the others as pawns and never reveals what his true intentions are. But, he chooses them due to a common thread- The island nation of Grim. Grim's capital of Feyreach was the first location to open a portal into the Fey Wild and thus makes it the staging ground of Salethar's ambitions.
Which divine sealed away his parents is unknown, but it's likely to be a diety under Zel, or even Zel himself.
While he never makes any progress in taking over the Fey (he was impeded by Warty, guardian of the Fey Wild), he eventually asserts himself as the ruler of Avernus in his father's stead. He tricks the other Archdevils by saying that he slew Asmodeus himself. He even frees Levistus from his icy prison in Stygia despite warnings of his volatile nature.