Bellatrix Lestrange, like her sister Narcissa Malfoy, was a Black before she married Rodolphus Lestrange. She is Sirius's cousin. In Book 4, Harry watched the trial where Bellatrix was convicted of torturing Frank and Alice Longbottom to insanity. She looks something like Sirius, with clear marks of great beauty that have now been ruined by years in Azkaban. Bellatrix is totally devoted to Voldemort; she is also an insane sadist.
In Book 5, Bellatrix is one of the ten Death Eaters to escape Azkaban and rejoin Voldemort. When Harry goes to the Department of Mysteries to find Sirius, Bellatrix is one of the Death Eaters who ambushes him in pursuit of the prophecy. She casts a Cruciatus Curse on Neville, clearly enjoying the fact that she is cursing the son of the two people she drove to insanity with the exact same spell. She is the one who Sirius is fighting with when he slips through the archway to the other side and dies. Harry, in revenge, chases Bellatrix through the Ministry of Magic intending to hurt her. She mocks Harry for attempting to kill her after she killed Sirius, but Bellatrix realizes that Harry is on the level when Harry Potter casts Crucio on her.
The Cruciatus Curse doesn't work (which says something about Harry's innate goodness). Bellatrix tells Harry the key missing element: "You need to mean [the Unforgivable Curses], Potter!" (36.32). And Bellatrix has no trouble meaning things like Crucio and Avada Kedavra, the killing curse.
Bellatrix appears to be afraid of one thing: her boss, Voldemort. When Voldemort appears in the Ministry, Bellatrix shrieks, "MASTER, I TRIED, I TRIED – DO NOT PUNISH ME —" (36.48). She is terrified that she will be held responsible for the lost prophecy. After Rowling has spent many pages convincing us of how hardcore Bellatrix is, with her Unforgivable Curses and her mad giggling, this evidence that Bellatrix is terrified of Voldemort emphasizes how scary Voldemort should be to us.