Bellatrix was a sadist and became a Death Eater and married Rudolphus Lestrange, brother of Rabastan Lestrange. The three of them plus Barty Crouch Jr used the Cruciatus Curse on Neville Longbottom's parents until they went insane. For that crime they were judged and sent to Azkaban but the three of them broke out and rejoined Lord Voldemort fourteen years later. The Lestranges are considered some of Voldemort's most devoted followers.
When she and a group of Death Eaters tried to steal a prophecy from the Department of Mysteries with Harry Potter, she spoke to Harry as the though he was two years old. She also murdered Sirius Black. She tried to take the prophecy from Harry Potter, but it had been smashed. She is currently residing with Lord Voldemort as of the end of Harry Potter and the Order of the Phoenix.