The myth of the Foretold grew up around what the Twelfth Doctor identified as an "ancient soldier being driven by malfunctioning tech".
Characteristics
The Foretold was injured in a war and fitted with "state of the art camouflage" and a "personal teleporter". The technology within it kept it alive, leading it to kill for thousands of years after the war, draining the energy from its victims to sustain itself.
The Foretold killed its victims sixty six seconds after first appearing to them. It took them out of phase in order to drain them, meaning only they could see it. The Foretold selected its victims based on physical and mental illness, preying on the weakest first. (TV: Mummy on the Orient Express)
History
The Foretold was brought on board the space-faring Orient Express where experts on alien mythology and the sciences had been brought together to study it without their knowledge. The Foretold first killed Mrs Pitt, then a chef, and a guard.
The computer Gus revealed to the Doctor and the passengers the reason why they had been brought together and presented them with a laboratory in which to work. There, the Foretold killed Professor Moorhouse, who was plagued by panic attacks, and Captain Quell, who suffered from post-traumatic stress.
It came for Maisie, but the Doctor extracted the essence of her mental illness and implanted into his own mind. This tricked the Foretold into believing he was Maisie. The Doctor identified it as a soldier and surrendered to it. Freed from its duty, it gave a final salute and disintegrated. The Doctor used its teleport to get the survivors off of the Orient Express before Gus destroyed it. (TV: Mummy on the Orient Express)