Maerorus is an Assassin temple of the Officio Assassinorum in Warhammer 40,000, although only a single assassin from this temple is known. Mentioned in the book Seventh Retribution, Legienstrasse was created at the behest of the Officio Assassinorum around M40 with the aid of Hereteks (as even the promise of an STC wasn't enough to convince the Adeptus Mechanicus to help out) and forbidden sciences, crossbreeding and xeno hybrids (likely Kroot or possibly Tyranids, but the latter is unlikely since she was made way before first contact on Tyran and it took a millennium to track her down, so unless they used proto-Tyranid DNA (perhaps from the Ymargl Genestalers?) they used the Kroot instead) were used in the creation of Legienstrasse. In her creation the ultimate heresy was committed: purposeful violation of the holy Human form. The purpose of this experiment was to create the ultimate assassin who would not need to rely on fallible technology, sending the agent in to beat everyone to death with their bare goddamn hands. While this seems as the Officio Assassinorum tried to weaponize Hokuto Shinken, the truth is far, far worse. She was able to absorb the biomass of a target upon touch, completely removing the target whole. She could use this new biomass to mutate at will or to create more Maerorus assassins by laying eggs (with a single clutch being able to kill an entire Imperial Guard company shortly after hatching), making her prime material for /d/. So in order to create the ultimate assassin, the Officio Assassinorum created an expy of Alex Mercer from Prototype, only a billion times worse because it's 40K (or they tried to make Alex Mercer even more grimdark by crossing him over with John Carpenter's version of The Thing).
It does not need to be said that this was not going to end well.
Legienstrasse figured that, as the most OP PLZ NERF person to walk the Imperium since
Colonel "Walking Exterminatus" Straken (he wasn't even born at that time) the Emperor himself, she didn't want to follow orders and escaped. The Officio Assassinorum tried to cover it up the same way they do everything (IE: kill everyone involved) and they began to chase her. Roughly a millennium later they cornered her on the planet Opis. Corrupting the planet's ruling class and collaborating with the World Eaters and Daemons to attack the planet, figuring that the ensuing war (and likely Exterminatus) would kill everyone involved. Sure enough, an Inquisitor and his gang rolled in to check the situation out. Afraid that their scheme would be uncovered, the Inquisitor and his posse suddenly died to Ork Berzerker snipers who were totally not Vindicare operatives.
Eventually a liberation fleet consisting of Imperial Guard, the Imperial Fists lead by Captain Darnath Lysander and a fuckton of Assassins (including two Grand Masters) dropped in. Sure enough, during their first encounter she managed to dodge sniper fire from Imperial Fists Scouts and a Grand Master of the Vindicare temple while fighting Lysander, a combination of Assault and Veteran Marines, the Chapter's Emperor's Champion and the Grand Master of the Culexus, Lady Syncella. Legienstrasse killed a number of the Imperial Fists and the Culexus operative.
During their second encounter (this time with Terminators, an Epistolary, an Eversor operative and once again the Emperor's Champion), the Grand Master of the Culexus temple and Lysander took the fight to Legienstrasse. She killed pretty much everyone but Lysander, but when she mortally wounded the Eversor Lysander backstabbed him with the Black Sword of the Emperor's Champion, drove the blade through the rogue assassin and pinned both of them to a pillar. The Eversor exploded, severely wounding Legienstrasse and giving Lysander the opening to cut her head off with the (still intact) Black Sword without uttering a one-liner (because he's an Imperial Fist, and they don't have any jokes). Somehow this did not do the job so Lysander took his Thunder Hammer and wailed on the headless body like he just picked up the Hammer item in Super Smash Brothers until all was dust.
Luckily the Officio Assassinorum learned their lesson and stopped all development of the Maerorus (or more likely they drew up plans for Maerorus 2.0 and said "okay, this time we don't give it free will, we put in that remote self-destruct, and everything will be fiiiine"). But nobody's entirely sure if ALL of Leigenstrasse's eggs were destroyed...