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.
