The central threat in ‘Mummy on the Orient Express’ is the Foretold. This supposedly mythical being can only be seen by victims. Once it appears the victim has 66 seconds before the mummy-like being touches them, killing them instantly.
Ultimately the Doctor discovers that the Foretold is a soldier from a long-forgotten war 5,000 years ago. The Doctor believes that he was wounded but was filled with equipment to keep him going.
Specifically it has a state-of-the art phase camouflage. This is why it is invisible to everyone and can pass through solid objects. It stays ‘alive’ by similarly moving its victims energy out of phase, this takes 66 seconds and is why they can see it. This draining effect makes it painful for the victim to look at the Foretold.
Potentially, since the phase shifting of the energy occurs at a distance, the Foretold doesn’t actually need to touch the victim to make the transfer. This can’t be confirmed because the Foretold also has a personal teleporter, able to move right next to the victim at the crucial moment.
The immortality resulting from this energy drain would be valuable technology. The only downside is that it kills the energy donor but there is no reason that it needs to come from sentient races. It is easy to imagine that large livestock could provide the same sustenance. Even if it does requiring intelligent races to work there are many who wouldn’t have a problem with that.
A whole society could be transformed by such an implant. This would create a vampiric race, feeding on others for energy. If the Time Lords were still around this could lead to a new crusade against vampires.
The downside is that the Foretold can’t turn it off. It continues to drive him to feed on others, no matter how weary he is. This could be a problem for any society that adopts them. Turning the implant off could be akin to suicide and prevented by the government or the rules of decent society. This could allow an adventure that explores the issues of the right to die.
The teleporter would appear to compensate for the slow pace of the Foretold. Part of its injuries could be to its leg and rather than fix the damage they just gave it the ability to jump to where it needed to be.
Through examination of the victims chosen by the Foretold the Doctor comes to the conclusion that it is picking off the weak. This includes those who are psychological issues and physical trauma.
Part of its technology must therefore be scanning, both physically and mentally, nearby lifeforms to decide who is next. The Foretold appears to have limited intelligence as the Doctor is able to fool it into think he is Maisie by copying her mental trauma and injecting it into himself.
The real question is why this might be. The Foretold is a soldier, programmed to stop if its victims ‘surrender’ which indicates that it views them as an enemy force. If it can target anyone why pick the weak? Wouldn’t it make more sense for it to target the strongest, thus weakening the enemy?
Tactically wounded or weak soldiers drain resources. It could be that the race that designed the Foretold wanted to prolong combat or only face the best of the best in battle. Thus the Foretold was actually helping them.
The Foretold was anchored to what the Doctor believes is a flag. Could it be that this cloth material has some kind of beacon that the Foretold can follow. Did his creators use this to control where he went? Could it be that this is just the last remnant of his race that he just follows through time.
The Foretold similarity to a Mummy is due to being covered in tattered cloth. When it was first created he might have totally been covered. These bandages might have had some healing prosperities, helped preserve its body or provided protection from spending extended periods out of phase.
His body is covered in symbols that are similar to those that can be seen on the flag. These could be for decorative purposes, provide a name and rank for the solider, identify what the Foretold is or be part of its programming.
Physically the Foretold appears humanoid. It is 5,000 years old but the uncertain timeframe of the space Orient Express doesn’t rule out it being human. Could there have been an Egyptian themed empire that posed such powerful technology that was eventually forgotten?
While the Foretold is eliminated at the end of ‘Mummy on the Orient Express’ there could be more. The legends of the Foretold could be so prolific because there are many invisible, shambling corpses in various corners of the galaxy. Certainly an army with the capability to turn a wounded solider into a killing machine that can last thousands of years would have done it more than once.
Of course it could be a prototype. The conflict in which it was wounded could have eventually led to the demise of its people. It would be the only one of its kind, without anyone to dismiss it (until it met the Doctor).
Yet some of the legends were correct in that there was a way to bargain with the Foretold. When the Doctor does this it disintegrates shortly afterwards. This would suggest that this couldn’t happen more than once.
It is possible that the creators of the Foretold do or did exist long enough to spread some information about their creation. If it was bound to the flag they might have lost track of it and tried to advise other races what to do if they encountered it. After thousands of years those words of warning were turned into half remembered myths.
It could be that the only reason that the Foretold disintegrated was because the Doctor said it was relived. Possibly others did surrender to it and the Foretold then left, awaiting new enemies.