In The Philosopher's Stone, Harry re-enters the wizarding world at age 11 and enrolls in Hogwarts School of Witchcraft and Wizardry. He makes friends with fellow students Ron Weasley and Hermione Granger, and is mentored by the school's headmaster, Albus Dumbledore. He also meets Professor Severus Snape, who intensely dislikes and bullies him. Harry fights Voldemort several times while at school, as the wizard tries to regain a physical form. In Goblet of Fire, Harry is mysteriously entered in a dangerous magical competition called the Triwizard Tournament, which he discovers is a trap designed to allow the return of Lord Voldemort to full strength. During Order of the Phoenix, Harry and several of his friends face off against Voldemort's Death Eaters, a group of Dark witches and wizards, and narrowly defeat them. In Half-Blood Prince, Harry learns that Voldemort has divided his soul into several parts, creating "horcruxes" from various unknown objects to contain them; in this way he has ensured his immortality as long as at least one of the horcruxes still exists. Two of these had already been destroyed, one a diary destroyed by Harry in the events of Chamber of Secrets and one a ring destroyed by Dumbledore shortly before the events of Half-Blood Prince. Dumbledore takes Harry along in the attempt to destroy a third horcrux contained in a locket. However, the horcrux had been taken by an unknown wizard, and upon their return Dumbledore is ambushed and disarmed by Draco Malfoy, who cannot bring himself to kill him. Dumbledore is subsequently killed by Snape, who finishes what Malfoy started.