Before this time, Sirius was to be found in a cell in Azkaban Prison, where he was serving a life sentence for killing 13 people -- one wizard and twelve muggles -- with a single curse. However, Sirius was innocent of this crime. Peter Petigrew, the wizard he supposedly killed, had staged his own death and blown up the street behind him, killing the twelve muggles, to make it look authentic. While in prison, Sirius discovered the whereabouts of Petigrew, and after twelve long years in Azkaban, he was able to escape by transforming into his animagus form, a large black dog, and swimming away from the island. Sirius headed to Hogwarts, where he knew Petigrew was living, in his animagus form of a rat, as the pet of Harry's best friend, Ron Weasley.

Sirius spent the majority of the school year hiding in the Shrieking Shack, a supposedly extremely haunted house on the outskirts of the village of Hogsmede. During this time, he was able to wander the school grounds as Padfoot and was eventually able to capture Petigrew, then known only as Scabbers, and take him back to the Shrieking Shack. There, Petigrew was exposed for what he truly was and was led back to the school grounds to be turned over to the dementors of Azkaban. Unfortunately, Petigrew again escaped, and Sirius, having no proof that he was actually alive, was forced to go into hiding once again. 

For the next two years, Sirius stayed safe and well hidden, first in a cave in the mountains surrounding Hogsmede, then in his parents' abandoned home in London. However, in book 5, Harry Potter and the Order of the Phoenix, Sirius left the safety of his home to fight Lord Voldemort and his Death Eaters in an attempt to rescue Harry and several of his friends. During this battle, Sirius and his cousin, Bellatrix Lestrange, a known Death Eater who had escaped from Azkaban some months before, engaged in a duel. It was during this fight that Sirius met his end. As he fought off Lestrange, Sirius fell backwards, through a mysterious arch hung with a tattered veil, never to return again. It is questionable as to what actually killed Sirius: Lestrange's attack or passing though the arch. Either way, the end result is the same.

Harry Potter and the
Order of the Phoenix