Soul Death
When Harry is told about the Come and Go Room, he also explores a little bit of the Room of Hidden Things, where he finds an obscure spell that can kill a person completely much like the AK, but it's not illegal, mostly because it's unknown to the ministry. He teaches the spell to Sirius so that he has something to do, with the thought that he can be trusted to keep him it secret and he has the incentive to get back in shape so that he can take out either the death eaters or even Voldemort himself.
During the fight at the ministry Sirius doesn't die, it just seems like he is and he's just knocked out and later revived.
Dumbledore duels Voldemort in the Ministry atrium. Dumbledore is knocked out by Voldemort, who sends an AK at Harry as the ministry people arrive. Harry collapses and all hope for the defeat of Voldemort is gone, when a big black dog sprints across the atrium, bites Voldemort's hand, causing his wand to go flying, and, before he can catch it again, the dog swivels and turns back into a man, grabs the wand, shoves it under Voldemort's chin and casts "anima mortis", which kills Voldemort, all his horcruxes and all his marked followers, and then he cradles Harry and weeps over his body.
Remus also runs in, followed by the rest of the order and the teens, and goes up to Sirius to offer some comfort, when he stops with a confused expression on his face, tilts his head as if to listen in, then runs to Sirius and grabs Harry's wrist, looking for pulse, but before he can find it, Harry coughs and weakly says "please tell me you got the number of that lorry."