Sirius Black and Bellatrix Lestrange break out of Azkaban together.

When Sirius finds out that Wormtail is still alive, and goes to break out of Azkaban to kill him, Bellatrix begs Sirius to let her help. She blames Pettigrew for the disappearance of her lord. Both of them want to kill Wormtail. Sirius wants him dead to avenge James. Bellatrix wants him dead to avenge Voldemort. Together, the two of them decide to team up. 

Bellatrix despises her blood traitor cousin. However, she’s disheartened by how the House of Black has been torn apart by the war, and believes she can convince Sirius to renounce the Light after being thrown in Azkaban without a trial.

Sirius despises his blood purist cousin. However, he’s singularly focused on killing Wormtail, and is willing to do whatever it takes to see him dead. Also, Bellatrix reminds him of Regulus. There’s a small part of him that feels guilty. Maybe if he had been a better brother, things could have been different. He hopes that Bellatrix can still be redeemed.

Sirius Black and Bellatrix Lestrange. Two cousins, torn apart by war, are brought together by a shared mission: Killing Peter Pettigrew.