In the past it was often of the Dark Lords pleasure to invade ...
Severus Snape: [to Harry] In the past it was often of the Dark Lords pleasure to invade the minds of his victims, creating visions designed to torture them into madness, only extracting the last exquisite ounce of agony, only when he had them beging for death would he finally... kill them. Used properly the power of Occlumency will sheild you from access or influence. In these lessons, I will attempt to penetrate your mind, you will attempt to resist. Perpare yourself!
[Harry sits up. Snape raises wand]
Severus Snape: Legilimens!