First conceived in the early years of M40, the Maerorus Temple was developed by Hereteks under the express orders of the Officio Assassinorum. Centuries were spent trying to gain leverage over Mechanicus Adepts to develop the illegal technology needed to create the Maerorus's unique assassins. Whole wars were fought and even an STC was handed over to the priests of Mars as payment for the tests and research needed.
Ok that explains "what in the Warframe-looking hell is this"
At some point Legienstrasse went rogue and decided she had the potential to topple the Imperium as she saw fit. Such was the threat she presented, the Officio Assassinorum willingly made pacts with daemons and Chaos Space Marines from the World Eaters to attack the remote planet of Opis, where Legienstrasse had taken refuge.
Based based based it looks like the one novel this assassin appears in (Seventh Retribution by Ben Counter) is about his fun OC and you know what? Since it's one novel that isn't part of a larger series with already built up lore, that's cool with me