Sutekh is an Osiran (originally, and often still, spelled Osirian), from the planet Phaester Osiris - a member of one of the most powerful races the universe has ever known. With domed heads and cerebrums like spiral staircases, the Osirans possessed extremely advanced mental powers, as well as being masters of guile and ingenuity. They are also very long-lived, certainly much more than 7000 years.
Sutekh - brother of Horus, the Egyptian god of light - was clearly one of the more powerful Osirans. But he feared that other intelligent life would grow to rival him in power; so he declared all life to be his enemy, and set out to destroy all the living creatures in the universe. He started out by destroying Phaester Osiris, and then left a trail of havoc halfway across the galaxy. Claiming to bring the gift of death, everywhere that Sutekh trod, he left nothing behind but dust and darkness. He was known as Set, Satan, Sadok, the Typhonian Beast, the Great Destroyer, the Bringer of Death and the Lord of Death. Horus, accompanied by 740 other Osirans, followed Sutekh's trail and eventually cornered him on Earth, in Egypt, where it took the combined might of all of them to defeat Sutekh. This war of the gods entered mythology, and affected the Egyptians so much that their entire culture was based upon the Osiran pattern.
Horus felt that, as dangerous as Sutekh was, it was against the Osiran code to destroy him - by doing that, they would be as wicked as he was. Instead, Horus imprisoned Sutekh in a pyramid at Sakkara, leaving him paralyzed in a chair, able to move only his head. The entrance to the chamber was guarded by an image of the Eye of Horus, with an eye that glowed red. The forcefield that Horus used to imprison Sutekh was controlled by a power source within another pyramid on the planet Mars, well out of Sutekh's reach.
During his imprisonment, Sutekh usually wears long robes and a mask bearing the Eye of Horus. But in his true form he has a long, grey snouted face, rather like a jackal, and bright green eyes. With his immense mental powers, he is able to project his image into the TARDIS, and he even has the power to interfere with time. This power is also transferred to his servant, who can reverse time temporarily - for example, to undo a fatal gunshot. Sutekh is one of the few beings in the universe which actually has the power to destroy the future - Sarah gets a glimpse of what the Earth would look like in 1980 if Sutekh were not stopped, and it is a barren, stormy, desolate world circling a dead sun.
Sutekh has the power to read minds, and he can display people's thoughts on the monitor in his prison. He can exert physical control over people using a green beam that he emits from his eyes - forcing them to kneel, or shredding their nervous system into a million fibers, or keeping them alive for centuries wracked with excruciating pain. Yet he also offers power to those who would serve him, and clearly he has a lot to give. Along with his mental powers, Sutekh also has a cytronic particle accelerator in his tomb. His robot servants are powered by cytronic induction and Sutekh also uses cytronic control to enable his communication with Scarman. Sutekh also seems to be fond of menacing organ music.
Sutekh remained imprisoned for thousands of years in his cavern with its pink and purple walls, eventually becoming the last of the Osirans. His only amusement, presumably, was a monitor which he could access mentally, linked to an instant data retrieval system which was displayed on the screen. It was not until the year 1911, when an archaeological expedition breached Sutekh's pyramid, that an opportunity for freedom arose. Dr Marcus Scarman, an Egyptologist, paid no heed to the symbol of the Eye of Horus and entered Sutekh's chamber. Immediately the monitoring system on Mars detected Scarman's entrance and triggered an alarm system, sending out the message "Beware Sutekh" on a radio frequency.
But it was too late - Sutekh was able to transfer his mind into Scarman's body, turning him into little more than an animated corpse with pale skin and red-rimmed eyes. Scarman essentially becomes something which is neither human nor god, the embodiment of Sutekh's power. It is as though Sutekh's essence has taken control of his body, and can remember parts of Scarman's life as though the memories belong to someone else - which, indeed, they do. Possession by Sutekh gives Scarman great strength, enough to effortlessly strangle a human.
When the relics from the tomb are returned to Scarman's house by his dedicated Egyptian follower, Ibrahim Namin, one of the sarcophagi is set up to serve as a combination communications device and space/time tunnel. One end of the tunnel is its physical location - in Scarman's old home, the Priory - and the other end is in Sutekh's tomb. Communications are begun by raising a hand, which causes part of the sarcophagus to begin strobing with color. When the tunnel is activated, the pull is apparently very powerful, and both things and people can be sent through it. The pull of the tunnel can be disrupted, although it is unclear how this is done - throwing something into the tunnel seems to work, and although this causes a small explosion in the process, the tunnel is not permanently damaged.
Sutekh sends Scarman through the tunnel to the Priory, to act as his eyes and ears. Initially Scarman wears black robes, gloves and a helmet with the same shape as that of Sutekh's mummy servants. Wherever he steps, the ground smokes - and the people he touches die in the same manner. In this way, Scarman is able to dispose of Namin and take his place as the only servant Sutekh needs. The robes, and the burning powers, disappear, leaving Scarman as nothing more than a zombie. Scarman begins to direct operations at the Priory, with the ultimate aim of freeing Sutekh from his tomb. Without this outside contact, Sutekh would be as powerless as when Horus left him.
Along with the communications/travel sarcophagus, Namin brought back other sarcophagi containing robot mummies. Scarman has a telepathic link with them, and uses them as servants to help him accomplish his tasks.
Scarman's first task is to set up four generator loops, shaped like canopic jars, around the Priory, creating a deflection barrier - a kind of forcefield - which makes the interior secure. Scarman then orders the mummies to destroy all the humans within the barrier, to eliminate any chance of Sutekh's plan being disrupted. The next step is to begin building an Osiran war missile, shaped like a large glass pyramid - which, despite its shape, is still quite able to fly (it "transposes with its projection", according to the Doctor). The warheads within the rocket are aimed at the pyramid on Mars - and if the pyramid is destroyed, the forcefield holding Sutekh will be destroyed as well.
The Doctor soon realizes that all this must be stopped - for what most people consider evil, Sutekh considers good. If Sutekh is freed, he will destroy everything in the universe, and even the Time Lords' power is not great enough to stop him. He first tries to break Sutekh's control of Scarman with an etheric impulse projected along precisely the right axis, but his device is destroyed.
His next resort is to destroy the rocket itself. In order to create a "door" within the deflection barrier, the Doctor disables one of the generator loops. He and Sarah are then able to find gelignite which they detonate within the rocket. However, Sutekh is able to halt the explosion using sheer mental power (although this is an intense toll on his available energy, and he cannot hold the explosion back for many minutes). Before the mummies can remove the gelignite, the Doctor takes a trip to Sutekh's lair - and by distracting Sutekh for just a moment, his control over the gelignite is weakened and it explodes.
With the rocket destroyed, Sutekh must send Scarman to the pyramid on Mars in person, and to do this he commandeers the Doctor's TARDIS. The control center where the Eye of Horus is housed is located in an antechamber under the main pyramid. To reach it, Scarman - followed shortly by the Doctor and Sarah - must make his way through a variety of Osiran traps which Horus laid for the unwary. Tracing the shape of a doorway with one's hands opens the first room; then the traveller is confronted with the traps. In the first room is a button which looks as though it would open the next door, but in fact it is connected directly to a power terminal; the real button is hidden behind a panel in the wall. The next room contains a puzzle where, if the incorrect answer is chosen, the floor will explode. In the final room, one of the travellers is trapped by a Decatron crucible while the other is confronted by the Twin Guardians of Horus - one will tell him the truth about which button makes the crucible disappear, while the other will lie and persuade the traveller to press the wrong button.
Once all of these traps have been successfully passed, the traveller is allowed entrance to the inner chamber, provided they make the sign of the eye in front of the door. Within the chamber is the Eye of Horus, an egg-shaped red gem held in a petaled stand. When the gem is destroyed, Sutekh is freed. First his essence departs Scarman's body, causing it to crumble into smoking black dust and then fade away to nothing. Sutekh remains trapped for the two minutes during which the signal is still travelling to Earth, but then his paralysis vanishes and he is able to move again.
Sutekh enters the tunnel and leaves his pyramid - but before he can exit on the other end, the Doctor uses the time control from his TARDIS to move the threshold of the corridor far into the future. Unable to ever reach the end, Sutekh is caught in the corridor of eternity and continues travelling forward in time until he eventually dies. Then the thermal balance equalizes and starts a fire, which burns down the entire Priory and everything within it.
