Krail was a Cyberman from the planet Mondas. He was one of the first wave of the Cybermen force which invaded the Snowcap Base in Antarctica in December 1986 with the intention of detonating a Z-Bomb in its missile silo. (TV: The Tenth Planet)
"Krail" was originally Eric Krailford, a young man and friend of Yvonne Hartley, living on Mondas. He was recruited by the Central Committee of Mondas to work as one of the "Surface teams" that worked on Mondas' propulsion engines on the surface of the planet. However, due to the freezing nature of the conditions on Mondas' surface, he was converted into a Cyberman to be able to work on the surface of Mondas, like all of the other surface teams. (AUDIO: Spare Parts)
Krail acted as Cyber-Leader, (PROSE: Doctor Who and the Tenth Planet, AUDIO: The Five Companions) and spokesman of the Cybermen and refuted the First Doctor's statement that emotions had value. When he and the other Cybermen of his wave invaded, Krail explained that they were Cybermen, and explained briefly about their background before Cutler activated a control which gave an emergency signal to Europe. Krail ordered Cutler to contact Europe and tell them that nothing further had happened and that all was well, but Cutler refused and was knocked unconscious by another Cyberman.
Krail forced Barclay into contacting Europe by saying that he would destroy communications with Zeus IV, and Barclay briskly did as he was bid. Then, Barclay and everyone else were allowed to try to help Zeus IV get back to Earth, but Krail claimed it was a waste of time and a forlorn conclusion as the pull of Mondas was too strong. After Ben Jackson tried to use a gun on the Cybermen, Krail took it from him, broke it with his strength and had him locked in the projector room. The capsule then exploded, and Krail used this to back up his point that Earth wasn't safe, and in order to save everyone, he required everyone's name and age as he intended to take them back to Mondas to become like them. He didn't see why he and the Cybermen should mind if everyone on Earth died.