The Dark Lord is a position. Anyone with enough power or ability can become the Dark Lord. When Dracula died, I'm assuming all the power he had collected over the years as Dark Lord was dispersed and people like Graham, Dario, and Dmitri inherited it at birth or their souls just had those natural abilities, similar to how Hector and Isaac had the natural ability to Devil Forge.
Soma on the other hand is just the reincarnation of Dracula's soul, which seems to have the natural power of Dominance. He is probably the best candidate to retake the Dark Lord mantle, which Celia seemed to know, but she had to have a back up just in case Soma didn't assume the mantle.