Of Wolves and Serpents

What if Snape survived Nagini's attack and Voldemort never fell? What if the Potion's Master was saved by a creature thought to have been extinct for a millennia? This story begins six months after the final battle at Hogwarts. Dumbledore is still dead. Harry Potter is alive but in critical condition, Voldemort is alive and licking his wounds in the aftermath. Rated M just in case.