Drathro

Drathro, called the Immortal by humans, was an L3 maintenance robot created by the Three Sleepers to watch over them and their secrets while they rested in suspended animation.

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Drathro was created in the Andromeda Galaxy and placed as guardian over the Sleepers in the underground Marb Station on Ravalox (i.e. Earth) until 2,000,000,000 CE.. He used the 500 Humans residing there, which he called "work units", to keep the Sleepers alive and he used two humans, Tandrell and Humker as his personal assistants.

Both he and the Station was powered by a black light converter on the surface. He also made use of 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, attempting to take his technology and use it as humans did in the past. She did not know, however, that the totem she saw as a sign of the gods was actually the converter, and when Sabalom Glitz and Dibber arrived to persuade her to destroy it, he locked them up.

The Doctor and Peri arrived on Ravolox out of curiousity and discovered that the converter had begun to decay already. Thinking the Doctor was from Andromeda, Drathro had the Doctor brought to him, thinking he was to return there.

Instead, the Doctor told him he was from Gallifrey and, recognizing the planet, the robot made him fix the converter, though the Doctor tried to explain that it needed to be shut down and that if not shut down soon, it would create the first black light explosion, which may or may not have destroyed 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, and therefore that the explosion killing everything didn't mean anything. When she lead her people to invade the Station, Drathro used his energy to kill Katryca and Broken Tooth. With the help of Sabalom 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. (DW: The Trial of a Time Lord: The Mysterious Planet)

As revealed later, the secrets had actually come from the Matrix and therefore belonged to the Time Lords. (DW: The Trial of a Time Lord: The Ultimate Foe)