Charles Augustus Magnussen and Mycroft Holmes have known each other for a very long time, and just as long vampire!CAM has been trying to lure him into his coven - unsuccessfully, of course. To complete the transformation a new fledgling has to be willing and Mycroft is anything but; even now he is barely able to resist the creature's attempts to seduce and control him, but if Magnussen were to claim him as his sire... Mycroft would be bound to him for all eternity.

The only problem is, now that CAM has Sherlock (who is completely unaware that vampires even exist) under his thumb, Mycroft may no longer have a choice in the matter. Not if he wants to keep his brother safe.


BONUS if you feel like it
When Mycroft was younger he did have a short affair with Magnussen - before he realized his anemia and memory gaps were not coincidental.