Sirius detesting dark magic all his life because of his family. Fighting anyone who uses it but one day in the middle of war, he uses dark magic. He slices his hand and let's the blood drip onto the ruined earth, muttering an incantation. Remus Lupin is being attacked by Fenrir's werewolf pack for not joining. They're a ways away but Sirius can just sense it and with one spell, he sends them all howling. Their fur (and skin once the full moon is over) is branded.
Coward.
