Sirius Expelled

Sirius Black screwed up. He told Severus Snape about Remus's secret. Now, he's been expelled from Hogwarts where he is forced to take the Dark Mark and join Voldemort's side of the war. As time rages on, he falls darker and darker. Maybe he'll escape. Maybe he'll join Voldemort for good. Maybe he'll die somehow. He doesn't know. All he knows is that he's starting to lose himself in a desperate attempt to survive. Everyone hurts him and nobody cares about him. The only person he can trust is himself because it's every man for themself in the Noble and Most Ancient House of Black.

Or

Sirius Black is forced to join the Death Eaters while being helplessly in love with Remus Lupin.