The Warden

The Warden was a tentacled creature that lived in a tank that controlled the atmosphere and gravitational levels within an ancient park located on the Moon in 1878. Captain Richard Haliwell and Emily Boyes-Dennison were captured by its robotic servants and forced to take a few daring tests.

The Warden was described as a deliberately stupid creature, only taking action when it was explicitly threatened, such as allowing the ships of the British Imperial Spacefleet to land in the park despite their lack of authorisation even if it wouldn't let them leave afterwards.

After the expedition gained access to the Citadel, the Fifth Doctor and Turlough — the only two able to communicate with it thanks to the TARDIS translation circuit — tried to reason with it to release the gravitational pull and let the ships leave, including the Phiadorans. They soon realised that, while it continued to obey its original orders, whether out of habit or deliberate conditioning, it apparently wanted to die after being abandoned for so long. In a flurry of bullets that cut off the power to the Warden's life-support systems, it was killed. (PROSE:  Imperial Moon)