After Scarman was killed by Sutekh in his tomb, a faithful follower brought the Mummies to Priory House in England in sarcophagi. They had been confined to a locked wing of the house but had been active for some time. They killed Collins when he entered the room. Namin activated them once more in order to search for and destroy Warlock and his rescuers, the Fourth Doctor and Sarah. After Namin was killed by Sutekh's puppet, the animated corpse of Scarman, the Mummies were used as workers and guards. They set up a force-field around the house and constructed an Osirian War Missile to free Sutekh from his bonds. They also killed several people on the property, including a poacher who attempted to interfere. They were presumably all destroyed or left behind.
The Mummies working for Horus were referred to as "guardians" and obeyed only his voice. They were identical to Sutekh’s Mummies except for gold-coloured bands around their heads and bodies. (DW: Pyramids of Mars)
The Mummies were powered by a cytronic particle accelerator in the shape of a pyramid and located on the back. They took voice commands from whoever they were programmed to obey. A special ring was used to activate them or countermand their orders. (DW: Pyramids of Mars) They were also extremely strong, able to manhandle humans with ease and could easily strangle someone.
Behind the scenes
According to The Brilliant Book 2011, a book that contains non-narrative based information; Winston Churchill encountered the Third Doctor at Omdurman in the Sudan on 3 September 1898, whilst the Doctor was battling what appeared to be an Osirian service robot.