The Seeds of Maerorus
Although the names of Eversor, Callidus, Culexus and Vindicare will strike fear among most of the foes - and members - of the Imperium, little is known about this institute of the Adeptus Administratum nowadays. Founded in extreme secrecy by Malcador the Sigillite and the six masters of the Clades during the early years of the Great Crusade, the Officio came into the limelight after their failed attempts at disposing of the wayward Primarch Horus. They are kept on a very tight leash and can only be invoked by a Terminatus order issued by the Senatorum Imperialus.
The Six Temples - descending from the founding Clades - have permission to make use of "any means or tools to get the job done", even if it means banned tech from the Dark Age of Technology or Xeno Artifacts. They each specialize in one particular method of assassination: Sharpshooters specialists in marksmanship and sniping in the Vindicare Temple; infiltration and impersonation chameleons in the Callidus Temple; drug-fueled berserkers in the Eversor Temple; and anti-psykers in the Culexus Temple. The two other temples are classified by the Inquisition who keeps a close eye on the Officio: the Venenum Temple and its poisoning experts; and the Vanus Temple and its intelligence-gathering tactical and strategic geniuses.
But in the early years of M40, the Officio recognized that, no matter how skilled the assassin, it becomes exponentially harder to kill subsequent targets who have been alerted by the initial kill. There was a need for a new breed of assassins, living weapons able to operate without equipment or support for a prolonged period of time and to kill masses of targets in a very short time with no weapon except their own bodies. To that end, the Officio Assassinorum spent centuries trying to gain leverage over Mechanicus Adepts to develop the illegal technology needed, fought whole wars and even handed over an STC to the priests of Mars as payment for the tests and research needed. Ultimately developed by Hereteks under express orders, the Maerorus Temple was born.
During this huge illegal experiment, thousands of prisoners were rendered down to create the genetic material required to make the first assassin. Forbidden technology, mutant cross-breeding and Xenos hybrids were combined to create the first - and last - Maerorus assassin, Legienstrasse. 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 able to kill an entire Imperial Guard company shortly after hatching. Soon realizing she had the potential to topple the Imperium, she escaped and went rogue.
Such was the threat she presented, the Officio Assassinourm willingly made pacts with the World Eaters to attack the remote planet of Opis, where she had taken refuge. Decades of civil unrest initiated by the Assassinorum resulted in an investigation by Lord Inquisitor Kekrops who died as soon as he landed on the planet. Full-out war was declared on Opis and an Imperial retribution force of dozens of Imperial Guard Regiments and a sizable detachment of Imperial Fist Space Marines under the command of Brother-Captain Darnath Lysander were deployed to bring Opis back into line.
Legienstrasse survived their first encounter, dodging fire from Imperial Fists Scout Snipers and the Grand Master of the Vindicare Temple while fighting Lysander, the Chapter's Emperor's Champion and the Grand Master of the Culexus Temple, Lady Syncella, killing a significant number of Assault & Veteran Marines. During their second encounter, after slaughtering almost all her opponents, she ended up being beheaded by Lysander wielding the Black Sword of the defeated Emperor's Champion. After almost a millennium on the loose, and with great losses to the Imperial Fists, the Imperial Guard and the Officio Assassinorum alike, Legienstrasse died, and with her the Maerorus Temple, and the ultimate heresy that was committed: purposeful violation of the holy Human form.
Although all life on Opis was exterminated and all evidence deleted, nobody can be entirely sure that all of Legienstrasse's eggs were destroyed...