The myth of the Foretold grew up around what the Twelfth Doctor identified as an "ancient soldier being driven by malfunctioning tech".
Characteristics
The soldier was injured in a war and fitted with "state of the art phase camouflage" and a "personal teleporter". The technology within kept him alive, but in order to allow him to continue "the war", it forced him to kill people every few minutes for millenia, draining the energy from his victims to sustain himself. He remained within the vicinity of a "scroll", which was in fact the flag under which he had fought. Eventually, his time-withered form, wrapped in the bandages originally used to heal his wounds, was confused for a mummy, and came to be referred to as "the Foretold".
The Foretold killed his victims by moving them out of phase, synchronizing them with his own phase level, a process which took exactly sixty-six seconds, during which time the victim - and only the victim - could see and interact with him. After precisely sixty-six seconds, the Foretold placed his hands on the victim's temples, draining their energy, and they dropped dead, seemingly of a heart attack which was in reality a side effect of the energy drain. Driven by some remaining scrap of the soldier's mind, the Foretold selected his victims based on physical and mental illness, preying on the weakest first so that the strong could survive. (TV: Mummy on the Orient Express)
History
The Foretold was brought aboard the space-faring Orient Express, where experts on alien mythology and the sciences had been unwittingly gathered to study it, and later forced to participate in the experiment by the ship's computer, Gus. The Foretold first killed Mrs Pitt, then a chef, then a guard.
Once Gus revealed to the Doctor and the passengers the reason why they had been brought together, he presented them with a laboratory in which to work. There, the Foretold killed Professor Emile Moorhouse who was plagued by panic attacks, and Captain Quell, who suffered from post-traumatic stress.
Finally, the Foretold came for Maisie Pitt, but the Doctor extracted her grief, torment, and resentment - the causes of her mental disturbance - and implanted them into his own mind. This tricked the Foretold into believing he was Maisie. The Doctor identified him as a soldier and "surrendered" to him. Finally freed from his duty, he gave a final salute and disintegrated. (TV: Mummy on the Orient Express)