In the dark and things comming out.
Dam man.. When I was little I had an imagination that totally scares me to the point of having difficulties in sleeping. One of them were *exactly* that: floating monsters in the complete dark, I remember spending hours staring into the dark, making sure everything's just my imagination. This, is the masterpiece of surreal horror.
Though I've never experienced a real hullucination before. The worst I had was when I woke up once to the sound of a huge group of shouting people *it seriously sounded like a medevil war was going on*, just to realise it is dead in the night and no one is around. Never happened again.