what to do with a trojan horse

A war is coming, and Alastor Moody knows enough about war that when he finds a spy in teenage Sirius Black, he makes a series of hard decisions. Under his family's strangling hold, Sirius has never had time to be afraid of death, and when he finds he could aid the war effort, he obliges. Marauders Era, oneshot, character study/slight canon divergence. Warning: abuse.