Legio Cybernetica

The Legio Cybernetica were a part of 40K's first and second editions, before the grimdark introduced in 3rd Edition and tech-heresy became a thing. They were the Imperium's vast armies of thinking war-robots, whom without direct command and programming were completely helpless.

The Cybernetica forces played a part during the Great Crusade, deploying massive war machines to assist the Space Marine Leigons. During the Horus Heresy, Horus swayed the Fabricator-General of Mars (and therefore the ultimate commander of Mechanicus forces) over to his side along with the entire Legio Cybernetica. When the Emperor won his phyrric battle against Horus, the surviving traitor Mechanicus forces fled back to the Eye of Terror, where they presumably hide to the 'present day' of the 41st millennium.

See also:

• http://wh40k.lexicanum.com/wiki/Legio_Cybernetica - Lexicanum's article on the Legio Cybernetica.
• http://warhammer40k.wikia.com/wiki/Legio_Cybernetica - The 40K Wikia's article on the Legio Cybernetica.