Forgeworld AdMech…Militant Ranks

The Skitarii, also known as the Tech-Guard, are the cybernetic military forces of the Adeptus Mechanicus. They are used to defend Mechanicus Forge Worlds, manufacturing installations elsewhere in the Imperium and often accompany high-ranking Tech-Priests and Magi as their bodyguards. Skitarii also serve as the armed forces that defend the Mechanicus' Explorator vessels that explore the galactic frontier. Skitarii safeguard Titans on campaign from boarding actions and prevent hostile forces from reaching the war engines' dead zone, where its main weapon batteries are useless. A number of Titan Legions maintain their own Skitarii detachments for this purpose, and if they do not, their Mechanicus masters will provide ample forces to protect the God Machines. The term "Skitarii" is formally applied by the Tech-priests of the Mechanicus to all of the military forces under their command, including infantry and armoured vehicles, save for the Titan Legions and the Knights. However, the term colloquially has often referred solely to the Adeptus Mechanicus' cybernetic infantrymen. http://warhammer40k.wikia.com/wiki/Skitarii


As the heavily augmented cyborg shock-troops of the Ordo Reductor faction of the Mechanicum, the Thallax differed from the Skitarii regiments both in their purpose and the unique degree to which their bodies had been reshaped by augmetics. The Lorica Thallax which encases them was developed from the same Power Armour technology which led to the development of Legiones Astartes battle plate, but taken down along an entirely different avenue of development. Requiring a robust human specimen, the Lorica encased the major organs, nervous system and cerebrum, but replaced the skeleton and limbs entirely with armoured mechanical systems powered by an internal reactor-core. The agony of this process, along with the replacement of the usual human sensory apparatus, proved so traumatic as to require the surgical excision of the pain centres and emotions, leaving the Thallax a cold, calculating killing machine, yet one that still retained a degree of independent human thought. However, for some within the Mechanicum this transformation of the human mind skirted the edge of abomination such as that posed by sentient "Abominable Intelligence" (A.I.). Given the resources required, the difficulty of creating a Thallax and the fact that an individual's will to live was a factor in whether a usable measure of sanity was retained in the subject, Thallax conversion was often reserved for suitable Forge Guard and Skitarii fallen in battle and subjects obtained from more mysterious sources by the unfathomable and baleful Ordo Reductor. http://warhammer40k.wikia.com/wiki/Thallax_Cohort

Knight Titans
A Knight is a small combat walker of the Adeptus Mechanicus that is one-man version of a Titan. Knights normally deploy in squadrons comprised of several of these war machines. The most common type of a Knight is about nine metres tall, with its head serving as the cockpit. Knights are the smallest and weakest of Mechanicus walkers, but they still are formidable and fearsome machines, all armed with an array of weapons beyond the power of even the strongest of Space Marines to bear. http://warhammer40k.wikia.com/wiki/Knights

Classiarii
The Skitarii forces of the Adeptus Mechanicus take a variety of forms. On warships and Explorator vessels operating within the Koronus Expanse and the Calixis, specially-programmed Skitarii are a necessity, for they serve as the members of boarding parties, security personnel and bodyguards where other forms of Skitarii would be impractical. Within the Mechanicus territories of the Calixis Sector, these warriors are known as Classiarii, a term of obscure meaning within the ancient traditions of the Calixian Mechanicus, believed to mean "mariners." Nigh-impervious to vacuum, fitted with thick, implanted armour, and equipped with an array of implanted weaponry designed for void combat and combat in the cramped enclosures of a starship, a Classiarius trooper is literally built for ship-to-ship combat
Venatorii
The Venatorii were created in the wake of the War of Brass that devastated the Skitarii Legions of the Lathe Worlds, leaving entire Forge Worlds of the Calixsis Sector with virtually no troops left. The reigning High Fabricator started a discreet rearmament drive throughout the Calixian Forge Worlds. The more military minded Tech-priests were moved into position to help replenish the fighting ranks. It took many years to bring this grand plan together, but once everything was in place, the High Fabricator found himself with a new force consisting of some of the best-trained and best-equipped troops in the Calixis Sector. Dubbed the Venatorii, the new hunters were a force that was to remain separate to the recovering Skitarii forces of the sector and act as the true elite of the Lathes. With their unique organisation and frightening red armour, the few outside of Mechanicus circles who were aware of their existence soon began to call them the Crimson Guard.
 




The character at the top is Magus Dominus.  The dreadnought sized unit pictured with the Thallax is called a Castellax.  No info that I can find on them as of yet.