The mind module was a component of the war machine created by the humanoid natives of a planet in the universe of superspace. It was the part which controlled the machine.
In 2267, the mind module was the only part of the machine to survive after the machine was destroyed by an exploding nuclear reactor. It began genetically altering the local plant life into an army of vegetable metalloids, which attacked any humanoids they found. At some point the mind module was discovered by Kyr Nostrand and made into part of a bracelet which he gave to his lover Rhuna shortly before his death. During a battle with the vegetable creatures in Caeminon, Captain James T. Kirk noticed that part of the bracelet glowed whenever they were near and shot it with his phaser, disrupting the mind module's control over them and ending the battle. Afterwards, he had the mind module fired directly into a nova, destroying the device and accelerating the star's collapse into a black hole, allowing the USS Enterprise to return to its own universe. (TOS comic: "Siege in Superspace")