Bellatrix is Sirius Black's cousin, and a member of the Black family. She was one of the three on trial alongside Barty Crouch Junior after the first wizarding war. She is proud of her continuing allegiance to Lord Voldemort, and attempted to find him after his downfall. Bellatrix is serving a sentence in Azkaban along with her husband and brother-in-law, Rabastan Lestrange.
Bellatrix is one of the Death Eaters who appears in the Department of Mysteries. She taunts Neville, reminding him of her torture of his parents. She tortures Neville Longbottom with the Cruciatus Curse, threatening to kill him if Harry doesn't turn over the prophecy. When the Order members arrive, Bellatrix duels against Sirius near the veiled archway. She hits Sirius in the chest with a curse, causing him to fall backwards through the archway, screaming triumphantly when he disappears behind the veil.
Bellatrix accompanies her sister, Narcissa Malfoy, to Spinner's End to see Severus Snape. She tries to persuade Narcissa not to visit Snape, as she doesn't trust him. She warns her sister not to talk about the Dark Lord's task for Draco in front of Snape. Bellatrix is suspicious of Snape's loyalty to the Dark Lord, and questions why he never tried to find him after he fell, and why he did not return immediately when Lord Voldemort summoned his Death Eaters. She is outraged when Snape reveals he returned on Dumbledore's orders, unimpressed by his attempts to spy for Voldemort whilst earning Dumbledore's trust. She is immensely proud that she served time in Azkaban for following Lord Voldemort.
Bellatrix is shocked when Snape reveals he knows about the task given to Draco by Lord Voldemort. She believes Draco has been granted a great honour, and doesn't share her sister's fears. Bellatrix is surprised when Snape agrees to make the Unbreakable Vow to Narcissa, and acts as the Bonder. She is silent, astonished, as Snape prompts her to perform the spell, stepping forward and placing the tip of her wand on Narcissa and Snape's linked hands. Her astounded face glows red in the glare from the spell as it is completed.
Bellatrix tortures Hermione at Malfoy Manor. When she threatens to allow Fenrir Greyback to attack the girl, she is shocked to see Ron and Harry burst into the room. She holds a knife to Hermione's throat to stop the boys. When she once again offers to give Hermione to Greyback, a peculiar grinding noise is heard, and the chandelier begins to crash onto her. She drops Hermione and throws herself out of the way.