The Hall of a Thousand Doors
It opened with a loud creak, echoing around the vaulted marble hall on the other side. Sirius stepped through the doorway and looked around for any signs of life. Nothing. He cast a last look back at the veil, rippling innocently far below, then closed the door with a bang, startling himself with the noise. The air, he noticed, was musty and thick with dust motes. The whole building gave off a general feeling of neglect, and Sirius instantly knew that nobody had been through the hall or in the stone chamber for many years. All around the hall were doors of different shapes and materials, some large and inlaid with gold; others very narrow and made of cheap wood. One that Sirius instantly recoiled from was a black void in the wall, bottomless as the night and adorned simply with a small silver handle.
Sirius chose one at random, mahogany with a large gold doorknob in the centre. He pulled it towards him, stepped forward and was confronted with a sheer drop down the side of a cliff. A black, storm-ravaged sea pounded the base of the rock and a gale screamed in fury, trying to drag Sirius over the edge. He yanked himself backwards and released the door, which slammed shut with a bang.
Breathing heavily and a bit shaken, he leant against the door and closed his eyes. "WHERE AM I, WHAT IS THIS PLACE?" He yelled, but no one answered but his own voice, echoing down the hall. The oppressive silence fell once again and Sirius felt the beginnings of panic stirring within him. He ran down the hall, trying to find a corridor, a flight of stairs, anything, but all he found were more and more doors stretching off into eternity.
Sirius collapsed onto the floor and buried his face into his hands. He felt something rise within him and without knowing why, threw his head back and howled, a loud primeval sound of anguish, which ripped from his throat. The change happened that quickly it was only when Sirius realised he was standing on four legs, that he had turned into a dog.