Oh, another one. I had a dream set in my high school, during exam time. Because of dream-logic, the school had invited a couple of venerable actresses (I believe Dame Judi Dench was one) to visit as a special treat, since we were all having our exams.
Anywho, I was late heading down to the exam hall (which was our gym building. That detail was accurate, actually). Somehow, I found out that the exam invigilator-- Mister Lucus-- had used the exam as a pretence to gather us all together in one building to kill us. Mister Lucus also had a shiny mech-suit, which made him infinitely more threatening. He chased me around the school for a while, along with the prefects (who were his minions. Dream-logic again).
Eventually, I gave him the slip briefly and made it back to the gym hall, and burst through a door at the back of the exam room to try to escape. That's when I saw the pool, full of this weird mist. I got onto my stomach to try to wave away the mist and see what was underneath... to find it was crawling with thousands of rats under the mist, crawling all over eachother. That's when Mister Lucus burst in to find me. It had been his plan to kill all the students in the rat pool. He even planned to kill Judi Dench.
Oh, and the scariest bit of all: the next morning after the nightmare, I headed to school, and my friend at school told me about a story she'd been writing, and the fucking villain's name was Lucus. I shit you not.