Handed the responsibility of using, maintaining, constructing and developing Battle-automata, they were bound by the Crimson Accords of Mars that forbade the creation of artificial intelligences that were termed Silica Animus. Fortunately, they didn't ban robots...er..."lesser and righteous works" of synthetic life. Well, basically robots that can be controlled.
|Fires of Cyraxus, where art thou?|
Unfortunately, the Mechancium was afraid of these awesome robots, having been bitten in the ass by the artificial intelligences, I mean Silica Animus that their predecessors created, they broke the Legio Cyberbetica down into smaller Cohorts. This was to prevent any one Archmagos from having too much power, and also to ensure all the supreme killing machines wouldn't just band together under one banner and proceed on a killing spree of their masters when they gained self-awareness a la Skynet. The Tech-priests have obviously watched and learned from the Terminator movies (except the horrible 3rd one and the awful Genesys, which we'll pretend never existed, I hope - Dark Fate was okay, if a little disappointing).
Commanded by an Archmagos or Magos Doinus, who would govern the war machines during battle, these senior Tech-priests were often accompanied by Myrmidons, enginseers, servitors, thralls and other personnel who would see to the Cohort's needs both on and off the field. The Cohorts also commanded their own support networks, mobile workshops, transports, battlefield vehicles, orbital landers and even star vessels - so you could field a Legio Cybernetica army with a Triaros Armored Conveyor, Krios Battle Tanks/Venators, Thallax shock troopers and Adsecularis Tech-thralls and still claim that it corresponds faithfully to the fluff. Unfortunately, the high costs of your giant robots mean you probably wouldn't have much space for mass Krios Venators or Triaros Armored Conveyor spam unless you're playing 3,000 points or more. Oh well.
Anyway, the Cohorts were forbidden to have their own forges, so they must rely on other Forge Worlds for supplies and repairs. This means a web of fealty and patronage, so the Forge Worlds they are dependant on could exploit them and command them to help out in their own wars and personal battles. They were valued greatly by these Forge Worlds and a myriad of Mechanicum masters and overlords, each wanting these formidable forces at their disposals.
This also meant a lot of support, so despite being difficult to construct and maintain, the Legio Cybernetica were able to build as many giant robots...ahem...Battle-automata as they want! Talk about owning your own toy factory...except that they aren't allowed their own forges so I guess they get to play with other Magi's toy factories.
So when Horus threw a tantrum and brought about his Horus Heresy, the Legio Cybernetica also split into Loyalist and Traitor forces, with the latter super-happy about abandoning the Crimson Accords and eager to pursue forbidden technologies. I would be too, except that I'm not very fond of research that see spikes growing out of my giant robots, or my flesh melting away from Warp-energies and a Daemon hungry for my soul. Yeah, I think I would rather stick with the Emperor and the Omnissiah.
After the Horus Heresy, the Legio Cybernetica continued and endured, and for some reason the Castellax Battle-automata and Thanatar Siege-automata all disappeared, only to be replaced by boring Kastelan Robots......sigh.