Complex sentence: After arriving at the Burrow, Harry has a vision of the wand-maker Ollivander being tormented by Voldemort, who claims that Ollivander had lied to him by claiming that the only way Voldemort could kill Harry was by using another person's wand.
Split into simpler sentences: After arriving at the Burrow, Harry has a vision of the wand-maker Ollivander being tormented by Voldemort, who claims that Ollivander had lied to him. Ollivander claimed that the only way Voldemort could kill Harry was by using another person's wand.