Heavily involved in attempts to accelerate the development of new Astartes for the Space Marine Legions during the Great Crusade, the Selenar who survived the First Pacification of Luna had sought ways to improve upon the Emperor's genetic designs for the Space Marines.
Unfortunately, all their efforts failed, and the supposed "Ur-legionaries" turned out to be mutants who were often mindless brutes as a result of the constant pain they endured. Some developed multiple eyes, limbs and even heads.
Even more disturbing, several of the Ur-legionaries developed facial features that closely resembled the appearances of the Primarchs.
After the project suffered from repeated failures, the Selenar decided to abandon the Ur-legionaries and admit that they did not know how to enhance the Emperor's original design.
All but 10 of the failed Ur-legionaries were euthanised and the gene-labs where the project had been pursued secretly beneath Luna's Herodotus Omega dome were sealed.
Recovery of the Magna Mater
Almost two centuries later during the climax of the Horus Heresy at the Siege of Terra, the mortally wounded Selenar gene-witch Ta'lab Vita-37 freed the remaining Ur-legionaries to help her prevent the Traitor forces of the Warmaster Horus from claiming the Selenar's Magna Mater relic.
She did so to defeat a squad of Sons of Horus Justaerin Terminators commanded by Captain Trastevere, who were assaulting her and the group of Loyalist Shattered Legions Astartes from the Iron Hands Strike Cruiser Sisypheum who had answered her call for aid broadcast into the void around Luna.
Despite their mutations, the Ur-legionaries were much larger and stronger than even the Justaerin Terminators and their altered Black Carapace implants allowed them to survive even multiple bolt wounds before dying.
So strong were the insane, raging Ur-legionaries that they were able to tear open the Justaerin's Terminator Armour as if it was no more than tissue paper.
Though the Justraerin ultimately prevailed over the unarmed and armoured mutants, only Trastevere and two other Terminators of the squad survived after killing all of the remaining Ur-legionaries.
However, due to the sacrifice of Vita-37 and the Ur-legionaries, the Shattered Legion Astartes commanded by the Iron Hand Captain Cadmus Tyro managed to escape from Luna back to the Sisypheum with the Magna Mater.
Though all of the Strike Cruiser's crew were later slain escaping from near-Luna space with the artefact save for Nykona Sharrowkyn of the Raven Guard, the Magna Mater would eventually fall into the hands of the Primarch Roboute Guilliman after the end of the Horus Heresy.
- Siege of Terra: Sons of the Selenar (Novella) by Graham McNeill, Dramatis Personae, Chs. 6, 10-11