I had this once, and never want to repeat it. Nothing to do with any of my mental issues.

One night years and years ago I realised I was dreaming, and was really pleased because I was aware in my dream world and so I could do anything (or so I thought). A simple test of my "power" backfired, and then I found myself in the clutches of this demonic thing, squeezing me. And it was like hag, wizened physically, like an old apple.

I kept trying to wake up, and kept getting flashes of my darkened room in front of my eyes, only to to end up back in the dream again. And the thing. It knew I was trying to get out, that I was trying to wake up, and it laughed. *shivers*

No nightmare before or since has ever equalled it for sheer malevolence, and I've had some nightmares.