5 years after their founding, the Legion Master took his forces on a "Crusade" out to the farthest reaches of space. Not even imperial records can explain exactly what happened, but 200 years after the crusade began, a transmission was received by Terra. It showed what appeared to be multiple large squid-like machines descending onto a planet, and vaporizing it with enormous laser weapons. At the end of the transmission, the words "We are the Mortis Reapers. The harvest has begun", and the Nova Terra symbol were displayed on screen.
Since then, the only known instances of these planet wide destructions have been few to none, and only on Xenos planets. However, each time one of these missions is complete, the ships retract all ground forces and retreat back into dark space.
Even their exchanges with their allies, the Nova Terra Regency, have been short. It was suspected that the Legion may have fallen to the Ruinous Powers, but this theory was disproved when the Legion engaged a Chaos Undivided fleet in orbit above a Nova Terran world unprovoked.
The Mortis Reapers make their home in dark space: the vast, mostly starless void between galaxies, aboard a fleet of highly advanced ships that havent been identified or classified by the Imperium. The exact number of ships is estimated to be between 25-40. Each Company makes their home on board one of the ships, and usually doesn't leave the ship unless ground engagements are absolutely neccesary. Their largest ship contains the Legion Master and his 1st company.
The R'tah Harvest: In M39, on an unknown date, the Mortis Reapers fleet descended upon a small Tau colony world and began to completely destroy the planet, leaving no survivors. It is believed that the Legion may have "Harvested", for lack of a better word, the planet's population.
The Nightborn Harvest: Mid M37, the fleet descended upon a smaller armada of Chaos warships belonging to a Chaos Undivided warband known as the Scions of Chaos. Within an hour, the Chaos armada had been expunged, and the planets controlled by the warband were razed.
First Fall: The first known defeat of the Mortis Reapers came at the hands of an Eldar Craftworld. The fleet encroached on the territory of the Eldar, and were warned that if they continued the approach, they would be fired upon. Ignoring the warning, the fleet began their usual Harvest protocols. Before the first ship made landfall, it was torn apart with a massive Eldar pulsar weapon named "The Breath of Khaine". The legion suffered minimal casualties, as they pulled back the fleet before this weapon could be turned upon them again.
The Legion's Gene-Seed is derived from that of Perturabo and the Iron Warriors. Their technological aptitude was inherited through the gene-seed, as the Mortis Reapers seem to have developed highly advanced technology when compared to that of other Legions. Whether or not this technology was made solely by them, or they had help, if not outright given, this technology from a Xenos species remains unknown.
They maintain no ties to their Primarch, or any other Primarch for that matter. They simply wish to establish themselves as their own legion, separate from the influences of any others.
The legion is divided into Capital Class Ships. Each ship contains 12,000 Astartes, over the span of 25 ships. Each ship, and by extension the company of Astartes within, are given a unique name.
Known Capital Class Ships:
Sovereign: Contains the Legion Master and his honour guards. The Astartes of Sovereign are the most elite, the best trained, and the most feared. Each of them has rallied a kill count of over 100 other Astartes, and countless Xenos.
Ascension: Contains the 11th Company. The 11th company itself is severely depleted of Astartes after their last encounter with the Astral Glaives. the Uren defense grid managed to puncture the armour of Capital Ship Ascension, venting many of the Astartes held in stasis within into the vacuum of space.
Zealot: Contains the 23rd Company. Zealot was one of the most recently manufactured Capital Class Ships, and as a result has not had much field testing. Most recently Zealot, and by extension the 23rd company, have been seen in the Segmentum Pacificus, seemingly searching for a world ripe for harvesting.
Migrant: Contains the 6th Company. Capital Ship Migrant has seen battles unending, having participated in both the R'tah Harvest and the failed Strion Harvest. The 6th company itself has seen very few ground engagements however.
Husk: Husks are the aggressive, mindless foot soldiers of the Mortis Reapers armies. They are created by impaling either living or dead humans on mechanical spikes that rapidly extract water and trace minerals and replace them with cybernetics. These cybernetics reanimate the lifeless flesh and tissue, transforming the bodies into horrifying killing machines.
The Mortis Reapers use large groups of husks to overwhelm the enemy. The husks' inability to feel pain, as well as their tendency to attack in groups, makes them particularly deadly adversaries.
The Legion in question has not recruited any neophytes since their induction into the Nova Terran regency. It is unknown how they maintain such a high number of Astartes without active recruitment, but the thought of a never ending force is truly an ominous one. While still under the control of the Imperium, they recruited mainly from feral worlds, as battle-hardened soldiers are better prepared to fight faster.
The Legion's fleet is comprised of massive and advanced fleet of Warships. These ships match no others in Imperial or Nova Terran records. Designated "Capitol Class", these ships take on the form of massive squid-like entities in the void of space. Traveling without Warp travel, these ships use a similar form of travel to Tau ships. The ships are all equipped with immensely powerful magneto hydrodynamic laser weapons capable of tearing other less advanced ships to pieces. Each of these Capitol Class ships has 6 leg (tentacle) -like limbs that can be deployed as functioning legs onto the surface of a planet, turning the ship into a highly effective, though extremely slow, Titan.
Rather than deploying the standard Firehawks, Assault Rams, and other aircraft, the Legion deploys it's own, most notably the Dart. The Dart is a fast-moving device used by the Mortis Reapers as a space and atmospheric superiority craft. Darts are cybernetic constructs, being piloted by a single Human servitor. Each Human is stripped down until only its nervous system remains, with the remaining tissue hardened to withstand vacuum and integration into a Dart shell. Darts possess powerful beam weapons, similar to other Mortis Reaper weaponry, and can invade an enemy ship by cutting into its hull after jettisoning part of its armor.
All members of the Mortis Reapers wear custom-made Mk IX 'Reaper' pattern Power Armor. Its most notable difference is the helmet, which contains but a singular ocular lens which glows an ominous red. Instead of standard issue Bolt Guns, each Astartes is equipped with a modified heavy Lasgun. This weapon is more potent than those standard issue weapons carried by the soldiers of the Astra Militarum, but less powerful than a Lascannon. They fire a continuous stream of Laser energy for 2 seconds, before venting for 1 second, and then being able to fire again.