The Kantrofarri, colloquially known as the dream crabs, were predatory creatures that fed on humanoid brain matter. Being blind and deaf, dream crabs hunted by using telepathy to pick up the perceptions of themselves in the minds of people. 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 an alien planet. More crabs then telepathically traced the Doctor's memories to Clara Oswald and attacked her along with four other people (Fiona Bellows, Ashley Carter, Shona McCullough and Albert Smithe). All but one of the victims were able to escape. (TV: Last Christmas)