Talk about it here.
The Kantrofarri, colloquially known as the dream crabs, were predatory creatures resembling misshapen clawed hands (when free-living) that fed on humanoid brain matter. In the absence of prey, they could lay dormant for centuries.
Being blind and deaf, dream crabs hunted by using telepathy to pick up the perceptions of themselves in the minds of people. The dream crabs could also see themselves by streaming the visual input from peoples' optic nerves to their brains. A crab would ambush its prey from above, wrap itself around the person's face and pacify them by placing them within a telepathically induced dream state. The crab then drilled into the victim's right temple and slowly liquefied the contents within for consumption.
Victims trapped by a dream crab could fight against the crab if they realised that they were dreaming. In turn, the crab could further attack the victim within the dream and place them within multiple layers of dreams to make escape more difficult. If a person were to successfully wake up from the dream, the crab would die and crumble to ash. Multiple people attacked by dream crabs simultaneously were able to share a common dream and interact with each other.
A dream crab attacked the Twelfth Doctor on a rocky planet. More crabs then telepathically traced the Doctor's memories to Clara Oswald in London and attacked her, along with Rona Bellows, Ashley Carter, Shona McCullough and Albert Smithe. By working together, all but Smithe were able to escape by waking up into the real world. (TV: Last Christmas)