Cataphractii Pattern Terminator Armour was originally a prototype for the later patterns of Terminator Armour first used by the Space Marine Legions and the warriors of the Legio Custodes throughout the Great Crusade and Horus Heresy eras. Whilst similar in many respects to the standard Indomitus Pattern Terminator Armour used by the Adeptus Astartes of the late 41st Millennium, it was set apart by a number of features such as the large, layered pauldrons, the pteruges protecting the elbow and thigh joints, and a helmet that resembled that used by the Mark III Iron Pattern suit of Power Armour. Although the use of the Cataphractii Pattern was rare before the Horus Heresy, some Legions, such as the Sons of Horus and the Iron Hands, possessed a large number of these suits. The Iron Hands Legion passed on these suits to their Successor Chapters during the Second Founding of the early 31st Millennium.
The technology for Exo-armour was originally developed during Mankind's Dark Age of Technology. These sealed environment suits enabled maintenance crews of spacecraft to operate in extremely hazardous environments such as hard vacuum or in other adverse atmospheric conditions. During the Great Crusade era of the late 30th and early 31 st Millennium, the Mechanicum was receiving a steady influx of new and rediscovered technologies from each planet brought to Imperial Compliance, and, in an effort to stem the horrendous loss of Legionary lives resulting from the ferocious conquests, attempted to design the "ultimate" pattern of Power Armour which would render a Space Marine virtually invulnerable. The design emerged from a hybrid of standard Legionary Power Armour, the cybernetic shell of a Dreadnought and the armoured hostile environment suits long worn by Terran engineers who worked in difficult environments like debris-plagued orbital space and the radioactive engine cores of interstellar spacecraft. Tactical Dreadnought Armour, as it was called, would turn the Astartes warrior within it into a living tank.
The eventual result of the Mechanicum's travails was only a partial success: the first suits of Terminator Armour did, as intended, afford a level of protection previously unattainable for its wearer. Like Power Armour, these suits were equipped with fibre-bundle synthetic muscles and imposed few movement restrictions upon the wearer despite their immense weight. But they were also too bulky, and their massive weight made running difficult despite built-in auto-balancers. The suits suffered from voracious power demands and few existing armaments could be carried as the armour's sheer bulk made handling and reloading impossible. But most importantly, the sheer amounts of raw materials and manpower required to craft Terminator Armour were so great that retrofitting the whole of the existing Space Marine Legions with it was simply not feasible for the already thinly stretched logistics of the nascent Imperium of Man.
Several patterns of Terminator Armour evolved in parallel from the Forge Worlds of the Mechanicum and the armouries of the Space Marine Legions. The Mechanicum refined their concept, and developed weapons specially adapted for it, like Combi-weapons and the Reaper Autocannon. Terminator Armour was then presented in limited numbers to the crusading Legions, who quickly found a use for them. Massively armoured, sealed against any hostile environmental conditions and incorporating their own heavy ranged or close combat armament, Terminator Armour designs soon proved their worth. Terminator Armour, with its ability to maximise the firepower and protection for a Space Marine, became standard on missions in extremely confined spaces like the corridors of a hive city, or during a boarding action on a Space Hulk where Dreadnoughts and armoured vehicles could not operate because they were too large to fit.
Anecdotal evidence of the adaptability and willingness to embrace new weapons of warfare can be found in the Warmaster Horus' vocal backing of the Tactical Dreadnought Armour project, with the result that his Luna Wolves Legion was one of the first and most widely equipped with Terminator Armour and at the forefront of the development of tactics for its use in assaults. By the time the Horus Heresy erupted and the Arch-traitor Horus first struck a blow against the Emperor during the Istvaan III Atrocity, these heavily armoured suits had become widely available to the Space Marine Legions. Cataphractii Pattern Terminator Armour was one of the first issued Tactical Dreadnought Armour patterns. The Cataphractii suits were even more heavily protected than their contemporaries, with slab-like ceramite pauldrons housing additional shield generators. This design had the unfortunate side effect of overstraining the armour's exoskeleton and slowing the wearer dangerously. This difference led to the pattern's declining use with some Legions at the outbreak of the Horus Heresy and the shift to the use of the Indomitus Pattern most widespread among the Space Marine Chapters of the present 41st Millennium.
- Cataphractii Primus - The Cataphractii Primus represents the very earliest example of Cataphractii armour, modified from suits which were designed to survive inside operational plasma reactors. While one of these suits is retained by the Martian Mechanicum and another in the Legion's own Forge, the third is traditionally worn by a Sons of Horus assault captain who has risen through the ranks of the Justaerin Terminator elite, its superior power generation and transmission systems granting the wearer protection far above that of other patterns of Terminator armour.
- The Horus Heresy - Book One: Betrayal by Alan Bligh, pg. 237
- The Horus Heresy - Book Two: Massacre by Alan Bligh, pp. 49, 57, 61, 75, 102, 144, 228, 249
- The Horus Heresy - Book Three: Extermination by Alan Bligh, pp. 70-71, 92, 119
- The Horus Heresy - Book Four: Conquest by Alan Bligh, pp. 154, 227
- The Horus Heresy - Book Five: Tempest by Alan Bligh, pp. 92-93, 112
- The Horus Heresy: Collected Visions (Artbook), pp. 33, 45, 52, 61-62, 65, 67, 78, 111-112, 138, 176, 189, 229, 235, 239-240, 243, 262-263, 274-275, 303, 352, 367