John Watson is a hundred and thirty year old vampire whose still maintaines a strong sense of humanity despite his darker history. When he meets Sherlock Holmes, a brilliant young human, and they move in together. Things soon come to a head as ever perceptive Sherlock starts to suspect something is wrong with his mild mannered flatemate, and John's old lover James Moriarty shows up, and also takes an unhealthy intrest in Sherlock.

I always see these as Vampire!Sherlock human John (Which I prefer) But that's why this idea intrigued me. The shoe on the other foot, and John and Jim being the arch enemies and powerful imortal beings, with a largely clueless Sherlock stuck in the middle.