This was a trade I did with  RainbowJokerHound of my werewolf Cyrus.

The spines in his back are from a mutation caused by a Wyrm inside his body. Other visible side effects tend to be violent/sadistic behavior and necrosis. Being a purebreed BSD grants Cyrus a bit more control over over the Wyrm, but not enough to ignore it's influence entirely. To prevent it from feeding on his own body, Cyrus needs to kill frequently, sometimes as many as four times a week to feed it's ravenous appetite.

The Wyrm has it's benefits as well, granting amazing bursts speed or strength, even making it's host numb to pain. But again, these benefits have their repercussions, often making the host push past their breaking point, tearing muscles or fracturing bone.

I'm well aware I probably got most of these facts about BSDs wrong, so WA players, give me a break^^;