Complex sentence: Harry and his friends, soon joined by members of the Order, enter a battle with the Death Eaters, during which Bellatrix Lestrange kills Sirius and Voldemort himself arrives to kill Harry, but Dumbledore rescues him and climactically fights the Dark Lord to a stalemate.
Split into simpler sentences: Harry and his friends, soon joined by members of the Order, enter a battle with the Death Eaters. Voldemort attempts to kill Harry, but Dumbledore prevents him and climactically fights the Dark Lord to a stalemate.