The Foretold

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 fitted with "state of the art phase camouflage" and a "personal teleporter". After he was injured, the technology within him kept him alive but also forced him to continue the "war" by killing others for sustenance. He remained within the vicinity of a "scroll", which was in fact the flag under which he had fought. His time-withered form, wrapped in the bandages originally used to heal his wounds, was eventually confused for a mummy and came to be referred to as "the Foretold".

The Foretold killed his victims by moving them out of phase, synchronising them with his own phase level. This process took exactly 66 seconds, during which time the victim — and only the victim — could see and interact with him. Once synchronised, the Foretold placed his hands on the victim's temples and drained their energy, causing them to drop dead of an apparent heart attack. 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 of interest to parties who wished to capture it and reverse-engineer its capabilities. On their behalf, the computer Gus exposed numerous ships to the Foretold and forced their crews to study it. These ships included the Gloriana and the Valiant Heart. (TV: Mummy on the Orient Express)

In her search for a Cyberman gauntlet that was located on the planet Pyro Shika, Sylen learnt of an ancient soldier driven on by malfunctioning technology, the Foretold. (PROSE: The Mondas Touch)

Over five thousand years after the myth of the Foretold arose, Gus brought its scroll onto the space-faring Orient Express, where the Twelfth Doctor and various experts on alien mythology and the sciences had been unwittingly gathered. The Foretold first killed Mrs Pitt, then a chef, then a guard.

Once Gus revealed to the Doctor and the passengers that they had been brought together to study the Foretold, 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 Hector Quell, who suffered from post-traumatic stress.

Finally, the Foretold came for Maisie Pitt, but the Doctor extracted her mental trauma and implanted them into his own mind. This tricked the Foretold into believing he was Maisie and targeting him instead. The Doctor identified him as a soldier and "surrendered" to him. Finally freed from his duty, he gave a final salute and disintegrated. The technology within him was left behind, which the Doctor used to teleport himself and the other passengers off the train before Gus destroyed it. (TV: Mummy on the Orient Express)

Whilst solving the mystery, the Doctor wrote a letter to Agatha Christie about his investigation. (PROSE: The Time Lord Letters) After the Doctor's mind was connected to a mind scythe, the "holiday snaps" the Doctor showed Kygon Brox included the Foretold. (COMIC: The Instruments of War)

Behind the scenes

 * He returned inside the TARDIS in the webcast The Doctor Surprises Fans At The Doctor Who Experience, which was shot at The Doctor Who Experience, in which he proceeded to attack everyone.
 * No explanation is given for why he is once again active despite his death at the end of his first meeting with the Doctor in TV: Mummy on the Orient Express; he can, at any rate, now be seen by all the characters in the scene, with no countdown in sight, implying that the nature of his undeath has changed significantly.