Caskelliac were a dark, slimy alien race with purplish skin who invaded Orkney, Scotland in 2025. They wore smokey silver armour studded with controls. Their head were lumps with no face recognisable in human terms. They had bristly tentacles on their heads and spoke with shrill, high-pitched voices. Their survival required that they conquered other races to use for energy. After harnessing the local power plants to fix all living things to the planet, they drained their life essences for survival. One of the race offered sympathy for those they destroyed for sustenance, but the Eleventh Doctor insisted there must be another way for them to sustain themselves. (AUDIO: The Ring of Steel)