The Warden was a tentacled creature that controlled the atmosphere and gravitational levels on the Moon in 1878. Captain Haliwell and Emily Boyes-Dennison were captured by its robotic servants and forced to take a few daring tests.
The Fifth Doctor and Turlough tried to reason with it to release the gravitational pull and let the ships leave, including the Phiadorans. They soon realised, as it had been abandoned for a while, it wanted to die. In a flurry of bullets, that cut off the power, it was killed. (PROSE: Imperial Moon)