Oh shit, just found out about curse of the moon, is there a reason to buy that? Doesn't like like they are tied together but the name, yeah?
They're very loosely tied together. Curse of the Moon isn't canon, but it does have 4 characters from Ritual of the Night as playable characters, and is probably a combination of Castlevania 3 and Rondo of Blood in terms of gameplay.
Fun as hell, and great music.