Master Brain

The Master Brain was the central computer that controlled the Land of Fiction. Having no imagination of its own, it fed on the creative power of the Master of the Land, usually via a cable connected to their head. The Thirteenth Doctor referred to it as the bookworm, and said that it was "An incorporeal, unknowable being. Not an evil thing – just one that needs stories to survive." (PROSE: The Wonderful Doctor of Oz)

History
At one point, the Master Brain enslaved a writer from 1926 as the Master and made him write until he grew old. The Master tried to replace himself with the Second Doctor so that he could leave, with the Master Brain controlling his mind and speaking through him in third person. It planned to take over Earth by turning Mankind into a "string of sausages," all the same, without free will. The Doctor freed the Master, and the Master Brain was destroyed by Jamie McCrimmon and Zoe Heriot overloading it. (TV: The Mind Robber)

After pulling a Cyberman invasion force into the Land, Zoe rebuilt the Master Brain and became the Mistress of the Land in order to fight the Cybermen. They sought to use the computer to remove imagination from humanity so they would be docile and conquerable, but were turned into fiction and winked out of existence. (AUDIO: Legend of the Cybermen)

Jason, installed by as the new Master of the Land, was connected to the master computer by a thick cable. (PROSE: Condundrum)

When Oz's palace began to crumble, Dorothy heard an inhuman, distraught, piercing sound, and realised that the Master Brain disliked how the story was going. She was attuned to its feelings because she was a "bookworm" herself. (PROSE: The Wonderful Doctor of Oz)

Appearance
During the Second Doctor's visit to the Land, the Brain appeared as a large, translucent, many-faceted orb situated among the Control Room's computers. It contained opaque webbing-like structures which pulsed with light as it slowly rotated. (TV: The Mind Robber) By one account, it was a "strange abstract object that stood at the heart of the computer display... A huge, whirled shape, imprisoned in a glass case: patterned like coral, and shaped like the kernel of a walnut, it pulsed with an inner light". This light changed between white, deep rose, crimson, a throbbing mauve, and purple; it finally turned black when overloaded. The Brain was also described as "floating". (PROSE: The Mind Robber)