Somewhere outhere on the internet is that epic fanfic about these two being separated at birth. Xanatos on the quest for power accidently freeing Ravencroft from Sarah's book (probably via time- or dimensional travel) and first tries to built a bond with his brother but evententually gets betrayed by Ben since he's even more power hungry and less loyal than David. And in the end there is that melodramatic fight of brothers who both seek immortality. I bet someone wrote that story and one day I'll find it, since I'd never be able to write it myself 😅