Breacher Siege Squad

A type of Legiones Astartes unit used during both the Great Crusade and Horus Heresy, Breacher Siege Squads were, as the name suggests, a specialized unit of Legion Tactical Marines geared up for sieges and fights within Zone Mortalis areas, though they were also used in void-warfare (especially when it came to ship boarding). Essentially, these guys were trained exclusively to fight in the most hazardous places the battlefield had to offer.

Clad in Void-Hardened versions of the MK.III Iron Armour, as well as armed with Boarding Shields and an array of assault and siege weapons, Breachers had better chances of survival and could reliably achieve their objectives (though considering how terrifyingly excruciating places like Zone Mortalis were even for the then young Legions, it says a lot). Some Legions (like the World Eaters, Imperial Fists, Salamanders and Iron Warriors being the most prominent ones) were geared up specifically for Zone Mortalis battlefields and had large numbers of these specialists.

In the modern, Post-Heresy Imperium, Breacher Squads were phased out, though it was part to the strains of extreme fights in the battlefields the Breachers operated that resulted in constant maintenance issues. This also what propelled the cog boys to accelerate the development of Terminator Armour.

Although the organisational unit of the Breacher Squad no longer exists in the 41st millennium; depending on the battlefield requirements, be it Zones Mortalis or acting in a Siege Vanguard, Tactical Squads can be issued with Boarding Shields or "Siege Mantlets" and largely fulfill the same role.

Specialist Breaching Formations

Phalanx Warders

In the Imperial Fists, breaching and siege units form a core part of their organisation and training, they may have even pioneered the use of Breaching Squads as a regular formation, as Horus was said to have noted their effectiveness in battle during the early Great Crusade and adopted the practice of arming some of his squads with breacher shields afterwards.

The Phalanx Warders were a company of marines who were assigned to the defense of the Phalanx directly, though they would be frequently dispersed to other fighting units across the galaxy to keep their skills well honed and to also provide an immovable line of defense and were most often found in close quarters ship-to-ship boarding actions which the Imperial Fists most often excelled at, indeed, the unique Imperial Fist Rites of War would always consider the Phalanx Warders to be a core part of their battleforce.

Though they would always carry a Boarding Shield, the remaining equipment that Phalanx Warders could be armed with can vary; either a boltgun or a power axe, depending on mission being performed.

Medusan Immortals

A specialized unit of Breachers that were unique to the Iron Hands Legion, Immortals were Iron Hands that were shamed by experiencing defeat or suffering grievous wounds (but not grievous enough to put them in a Dreadnought) and who were given Breacher equipment along with very extensive cybernetic reconstruction. Pre-Heresy Iron Hands had a very rigid and extreme set of standards that promoted strong-willed individuals and weeded out weaklings. If a marine was, as mentioned before, shamed in either of the above ways, the Primarch would strip them of any ranks and titles, give them a serial number, remove them from their original companies to fight on their own in what was essentially as living marks of shame.

For such marines, being an Immortal was the only way to gain redemption in the eyes of their Primarch and brethren. A formation that was described as Forlorn Hope and final chance at glory, the Immortals fought with determination in warzones so extreme that even standard Breachers would have had little chances at survival.

During the Horus Heresy, the ranks of Medusan Immortals swelled dramatically after the Dropsite Massacre as the marines there saw as their greatest failure and shame the death of Ferrus Manus and the Clan Council. A failure they couldn't repent no matter what. However, those who came from Clan Morragul were allowed by Autek Mor to fight alongside their brothers (which was actually unusual). And unlike other Immortals, they were allowed to wear a red talon symbol to further distinguish themselves (and these marines were the base on which the Red Talons Chapter would be founded).

Invictus Suzerains

As a precursor to what would become the Honour Guard of the 41st millennium, the Invictus Suzerains were the elite troops of the Ultramarines legion who were chosen from the ranks of other squads who have been singled out for potential promotion to command roles. It was their duty to act as the embodiment of their Primarch's will and served either him or his Tetrarchs directly, therefore they acted as a veteran cadre that could be attached to units in any battlezone.

Their wargear was symbolic, but undoubtedly powerful. They were gifted with Artificer Armour and Boarding Shields to represent them holding disorder at bay, and came with a Legatine Axe with which to enact swift justice, therefore they also acted as arbitrators and civilian peacekeepers.