I could see this happening with an outlander drow from the Forgotten Realms- here's one possible origin- the future darklord might've been raised by refugee drow who worshiped Eilistraee and taught their daughter to do the same. But perhaps the daughter felt drawn more to the darkness of her ancestors and saw the benevolence her parents espoused as weak. Her family fled to the surface, perhaps the forest of Cormanthor, because of their aberrant (by drow standards) beliefsedised wrote:a drow dark lord, but unlike other drow heard the call of good, had a chance to change and said no.
also became consumed with the drow ruling the surface, and loves the night sky and hate/envy of other elves. the dark powers her and she became dark lord of a elven land ruled by drow, with other surface elves hunted or subservient. the twist is the drow follow a good god (the one she rejected)and the other elves worship her evil drow god.
The daughter felt her beliefs were justified in the end when her parents were murdered, though she escaped, perhaps by betraying her parents. Still, the future darklord found she could not return to drow society, as the children of traitors are not accepted, nor could she make it in surface society as a singular drow with no further connections.
However, she eventually found a surface group of evil drow that venerated the evil gods of the Dark Seldarine. Thus the future darklord found new solace in the hands of an evil god, like Vhaeraun, Ghaunadaur, or more likely, Kiaransalee, the drow goddess of necromancy and revenge that hated both Lolth and the surface elf Seldarine. The future darklord went on to commit atrocities and provoke wars against both Lolth-worshiper drow and surface elves, stoking enmity toward each other, until she was taken away by the Mists.