Severus Snape: It appears there is a connection between the Dark Lord's mind and your own. Whether he is, as yet, aware of this connection is for the moment unclear. Pray he remains ignorant.
Harry Potter: You mean if he knows about it then, he'll be able to read my mind.
Severus Snape: Read it, control it, unhinge it. In the past it was often the Dark Lord's pleasure to invade the minds of his victims, creating visions designed to torture them into madness. Only after extracting the last exquisite ounce of agony, only when he had them litterly begging for death would he finally... kill them. Used properly, the power of Occlumency will help shield you from access or influence. In these lessons I will attempt to penetrate your mind. You will attempt to resist. Prepare yourself! Legillimens!