She was tasked by the Selenar High Matriarch Heliosa-54 with protecting the greatest artefact of Selenar genetic engineering technology known as the Magna Mater from being seized by the forces of the Emperor during the First Pacification of Luna at the start of the Great Crusade in ca. 798.M30.
After giving her the artefact, the high matriarch erased all of her own memories of her close confidant Vita-37. Vita-37 succeeded in protecting the artefact for over 200 Terran years until the start of the Siege of Terra at the Heresy's climax.
During the assault of the Traitors, Luna was conquered by the forces of the Warmaster Horus. He sought to claim the Magna Mater for himself and hoped to use it to expand his own forces of Heretic Astartes.
Desperate to avoid the capture of the relic by forces even worse in the eyes of the Selenar than those of the original Imperium, a desperate Vita-37 broadcast a vox distress call into open space hoping for Loyalist aid.
The Traitors pursued and in the resulting firefight, the gene-witch was mortally wounded by the Justaerin's assault. Knowing that her death was imminent, Vita-37 reluctantly entrusted the Magna Mater to the Astartes of the Sisypheum.
To aid their escape, she unleashed the 10 surviving Ur-legionaries upon the Justaerin, a group of mutated Astartes unsuccessfully enhanced by the Selenar in an attempt to improve existing Space Marine gene-lines.
The Ur-legionaries were able to overpower and kill most of Trastevere's squad before they followed Vita-37 into death.
However, this aid proved to be enough to allow Tyros' Astartes to escape with the precious artefact aboard the Sisypheum.
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, Prologue, Chs. 6, 8, 10-11