It's real name is Kantrofarri, but it's something I don't ever want you to have to encounter.
Think of a creature that's the shape of a clawed hand that grabs your face. Then it bores a hole into your right temple to slowly turn your brain into mush, which it eats. If you realize you're in a dream and wake up, the Dream Crab dies by crumbling into dust, so to prevent that it telepathically sedates you by feeding your mind a dream so you don't realize you're even dreaming and know you're in danger. You may force yourself to wake up, but it would be a dream within a dream. Layers of dreams. Tricky bugger.