Drathro

Drathro, known as the Immortal by a group of humans, was an L3 maintenance robot created by the sleepers to watch over them and their secrets while they rested in suspended animation.

Biography
Drathro was created in the Andromeda Galaxy and placed as guardian over the Sleepers in the underground Marb Station (part of UK Habitat) on Ravalox (i.e. Earth) until c. 2,000,000. He used the five hundred humans residing there, whom he called "work units", to keep the Sleepers alive and he used two humans, Tandrell and Humker, as his personal assistants.

The station and he were powered by a black light converter on the surface. He also used an L1 robot and a train guard named Merdeen, who helped humans to escape to the surface without Drathro knowing.

Queen Katryca had been planning to destroy Drathro. She hoped to take his technology and use it as humans had in the past. She did not know, however, that the totem she saw as a sign of the gods was actually the converter. When Sabalom Glitz and Dibber arrived to persuade her to destroy it, she locked them up.

The Sixth Doctor and Peri arrived on Ravolox out of curiosity and discovered that the converter had begun to decay. Thinking the Doctor was from Andromeda, Drathro had the Doctor brought to him.

The Doctor told him he was from Gallifrey and, recognising the planet, the robot made him fix the converter, though the Doctor tried to explain that it needed to be shut down soon, lest it create the first black light explosion, which might destroy the universe.

Drathro would not shut it down, however, feeling that since he was put in charge of the humans, they would have no function without him. The explosion killing everything wouldn't mean anything. When Katryca led her people to invade the Station, Drathro used his energy to kill Broken Tooth and her. With the help of Glitz and Dibber, Drathro was lured into letting the Doctor shut down the converter. As a result, Drathro and the box of secrets that he was carrying were destroyed. (TV: The Mysterious Planet)

As revealed later by Glitz, the secrets had actually come from the Matrix and belonged to the Time Lords. (TV: The Ultimate Foe)