Surrounded, but alone in the depths of the Forbidden Forest. So deep in yourself that you don't realize that something is coming near you. Or rather, many somethings. But they see you; and your fear in them, as soon as you realize they're there.
You try to run, but your feet seem to be bound by some invisible spell. You try to reach for your wand, but you can't reach it. You try as hard as you can, but you can't reach it. You are completely bound. You can't move, but they do. They keep advancing on you as you watch, fear running through your veins. You try to scream, but you make no sound. Nothingness creeps upon you from every bone in your body. Fear is the blood that runs through your veins. Again, you try to move or scream or do anything that might stop the nothingness. Or at least slow it down.
The pain is unbearable, excruciating. But you realize that the pain is more emotional than physical. Suddenly, everything bad that's ever happened to you is replayed in your head. A slideshow of your worst memories, the ones you'd hoped to have forgotten. The hooded creatures keep advancing on you, one nearly in front of you now. Your worst memory, the screams and the madness, is repeated over and over again in your head.
The nearest of them slowly takes off his hood, replaced by a black oblivion. Your memory pauses as sweat trickles from your forehead. With each new moment, the fear reigning upon you. Your last scream, heard by none but yourself, emptying you. Your heartbeat nearly over-powers the memory, but instead they stay perfectly in tune, each attempting to overpower the other. Putrid air breathes closer to you, suffocating you in the emptiness. The Dementor's kiss.