DEATH HAVE NO MASTER Harry Potter X Sweeney Todd Crossover (read description)

Snape survived and was saved by Wormtail on the orders of the Dark Lord. Voldemort told Wormtail to magically transfuse his blood into Snape. This was to make him a horcrux in case the Dark Lord death. It saved Snape. Somehow it worked, but Snape's guilt at still being alive, drove him to a suicide.