Complex sentence: Amidst the chaos, Bellatrix Lestrange kills Sirius and Harry faces Voldemort, who unsuccessfully tries to possess Harry in an attempt to get Dumbledore to kill the boy.
Split into simpler sentences: Amidst the chaos, Bellatrix Lestrange kills Sirius and Harry faces Voldemort. In the midst of the duel, Voldemort unsuccessfully tries to possess Harry in an attempt to get Dumbledore to kill the boy.