Skitarii

The Skitarii (singular Skitarius, also known as Tech-Guard) are the private army of the Adeptus Mechanicus. Under the Treaty of Mars, Forge Worlds are not required to give manpower over the Imperial Guard under the Imperial Tithe, so they are allowed to raise, train, and equip their own Planetary Defense Forces. Standing somewhere between the Guard and the Space Marines in terms of ability, the Skitarii have access to absolutely everything the Adeptus Mechanicus can create. The only things they don't use are the Land Raider (which is due to a decree by the Emperor reserving it for the Adeptus Astartes during the Great Crusade) and Dreadnoughts (though in theory they could). Like the Guard, there is little standardization among Skitarii units. For example, they can't quite seem to agree if the greater unit is called a cohort or a brigade. Additionally, Skitarii will vary from simple gun-servitors under a few commanders to gene-enhanced augmented humans with major equipment. Their brain is altered to maintain their loyalty and obedience. Some are just psycho-surgically or biochemically altered, while others are full-on lobotomised. On the plus side if they have a working brain left, it has have high levels of multitasking: a Tech-Guard officer will proficiently chainsword-duel with you, while happily chatting with his fellow Skitarii on the radio in binary, catch up on his unread messages, recompile the kernel of his cybernetics in head and troll all the boards of future-*chan in the same time.

They are generally the first line of defense on Forge Worlds and other Mechanicus installations. However, the also serve as marines (in the traditional sense) for Explorator ships, and act as dedicated infantry and vehicle support for Titan Legions when they get deployed.