It could be interesting to 'discover' that Marius is older than he looks, and that by feeding on someone, he can muck with their memories (so that his 'father' remembers him as a son (that he killed and replaced), and not a distant ancestor who has been creeping around the family for many generations).

Establishing him as super-old could then tie him to another super-old vampiric mutant, who has already shown a predilection for grooming young mutants of an amoral nature and gifting them with a smidge of her immortal vampiric nature...

And so Selene becomes the author of some of Monet's biggest woes, despite possibly never even having met her. "Oh, something I did on a whim a century or so ago, has ruined your family's lives for generations? How wonderful!"