Voldemort, as punishment for Quirrell's failure to secure the Stone from Gringotts, decided to keep a closer watch on his new servant. He took possession of Quirrell's body, which caused his face to appear on the back of Quirrell's bald head. To conceal this, Quirrell took to wearing the turban. To keep Voldemort alive, Quirrell had to kill unicorns in the Forbidden Forest, drinking their blood to provide temporary life-sustaining powers for Voldemort. Quirrell then reveals that he was the one who let in the troll and attempted to throw Harry off his broom while Snape muttered a counter curse. During the climax of the story, as Harry and Quirrell try to recover the Stone from the Mirror of Erised, Voldemort reveals himself on the back of Quirrell's head, formerly concealed by the turban, and speaks directly to Harry, threatening to kill him if he does not assist Voldemort in recovering it. After Harry refuses, Quirrell tries to take the Stone by force. Voldemort orders Quirrell to attack Harry, who holds off Quirrell long enough for aid in the form of Dumbledore to arrive, at which point Voldemort flees, in his non-physical form. Voldemort's departure, as well as the agony suffered by him because of his contact with Harry, who continues to be protected by the love of his mother and the spell that was cast by her sacrificing her life to save Harry, causes Quirrell to die, hence Dumbledore's comment that Voldemort is as merciless to his followers as to his enemies.