Existing D&D characters is tough because they should generally be tragic villlains who brought their curse down upon themselves, a decent backstory that can be discovered by PCs, can be big bad guys, and present an interesting theme for a domain.
Ravenloft has had only four in the big setting that I can think of, Strahd from I6 Ravenloft, Azalin from I10 Ravenloft II, Lord Soth from Dragonlance, and Vecna from his artifacts and then Greyhawk.
And I don’t really consider Vecna a great fit, he had the whole Kas betrayal but otherwise he was not that gothic a gothic horror villain and his land was more high powered evil lich cult fascism land than someplace for gothic horror adventures.