the shreiking shack!!

Falsely rumored to be "the most haunted building in Britain," the Shrieking Shack is a favorite stop on any Hogsmeade visit. However, the wails that were once heard emanating from this creepy building were not in fact ghosts, but rather Remus Lupin transformed into a werewolf. The shack, in fact, was built when he came to Hogwarts, allowing him to transform a safe distance away from the other students. The shack cannot be broken into from the outside, though Fred and George have tried; it must be entered instead through a secret passageway, the mouth of which lies under the Whomping Willow. It was in this building that Harry first met and learned the identity of Sirius Black, and also where he later watched Voldemort kill Severus Snape