Moony, Wormtail, Padfoot, and Prongs and the Invisibility Cloak

Sirius Black is a Pureblood wizard who has a troubled life.
His father died foolishly, his brother dislikes him, and he's best friends with a Half-Blood.
Sirius' family hated Mudbloods and Half-Bloods. If you weren't pure, you were a lower class.
Sirius would be turning 11 soon and getting ready to go to Hogwarts, there are 4 houses; Gryffindor, Hufflepuff, Ravenclaw, and Slytherin.
Sirius' family were all Slytherins, so if he didn't get Slytherin, he'd be a disgrace to the Black family.