Complex sentence: It is finally confirmed that this spell was created by Severus Snape, who is described as knowing more curses and jinxes when he arrived at Hogwarts than most did when they left.
Split into simpler sentences: It is finally confirmed that this spell was created by Severus Snape. Snape has been described as knowing more curses and jinxes when he arrived at Hogwarts than most did when they left.