He had finally relaxed when he watched them both stand in front of the Mirror Erised, neither had seen anything, being truly happy in their lives. He had been able to subtly use Legilimency on the twins, and from the very brief look he got, they had been telling the truth. Unfortunately, they both seemed to be proficient enough in occlumency, like many pureblood students, that they would feel him, fight him off, and possibly identify who was using it against them if he went deeper than surface emotions and reactions. Then the Philosopher's Stone went missing! Henry Potter and Ronald Weasley had done their job, defeating Quirrell, who he had dosed with his own creation. It was a potion that worked similarly to the Imperius Curse. It gave the potion maker complete control over the drinker, and it couldn’t be fought off like the Unforgivable. It also made the drinker extremely suggestible, small whispers in the ear became their thoughts and memories. Because of that, he was able to manipulate Quirrell into believing he actually had Voldemort on the back of his head and he was working for the wizard trying to restore him to a body. With a little basic illusion spell, Henry believed the same thing when he confronted Quirrell.