Maerorus was (very hopefully) 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 (for some reason named after a train station outside Hamburg) 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, and Tyranid) were used to make 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 like 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 un-godly emperor damned super abomination of Alex Mercer from Prototype and John Carpenter's version of The Thing, only a billion times
worse better because it's 40K.
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 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, the Assassinorum figured that the ensuing war would force her out in the open, allowing them to confirm a definite kill, along with the likely Exterminatus removing all evidence. Most alarmingly, she was able to take control of the both the daemonic and World Eater forces, seemingly without being corrupted by Chaos along the way. Predictably, 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 Berzerker snipers who were totally not Vindicare operatives.
Eventually a liberation fleet consisting of Imperial Guard, the Imperial Fists lead by Brother-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 Eversor 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 FINE!", which would probably only work for a time then end like this). But nobody's entirely sure if ALL of Leigenstrasse's eggs were destroyed...