The Officio Assassinorum (Office of Assassins) is the Imperial organisation responsible for the recruitment, training, and deployment of elite assassins.
The Officio's head is the Grand Master of Assassins and due to the power of his organisation, and by tradition, he is always a member of the Senate of the High Lords of Terra and, in theory at least, the agreement of the Senate is required for the deployment of assassins. Targets are key figures, such as rebellious Imperial Governors, heretic prophets, powerful demagogues, successful Xenos leaders, etc. The Officio consists of different "Temples"; each temple is unique and specialises in its own methods of killing. The heads of individual temples are called "directors." The motto of the Officio Assassinorum is: "For those that defy the Imperium, only the Emperor can judge your crimes. Only in death can you receive the Emperor's judgement."[Needs Citation]
Assassins are recruited at a very young age, from the orphans of Imperial servants raised in the Schola Progenium, and from the populations of feral worlds. They then undergo ten years of intensive training at the School of Assassins on Terra. From then onwards they live and operate from one of the Assassin temples said to be located somewhere on Terra. They are harshly tested and trained, their senses and resolve heightened with implants. Even after the initial training, the life of an agent of the Officio remains hard and unforgiving.[2a] Some Inquisitors are known to organise Officio Assassinorum training for Death Cultists that serve in their retinue which transforms these already highly efficient killers into much more deadly and honed executioners.
The agency is almost as secretive as the Inquisition itself and it is not surprising to see that the two organisations work closely with one another. The location of the Officio's headquarters on Terra is secret, and the Master of Assassins is never seen by the Adeptus Terra, though he is rumoured to have access to the Emperor himself. Typically, the Grand Master's loyalty to the Emperor must be beyond doubt, as he holds power that could feasibly topple the Imperium itself.[2b]
The ranks of the Officio Assassinorum also include ancillary staff, along with Astropaths, Navigators, and other such servants. Though these individuals are not Assassins themselves, they live entirely within the organisation and have no contact with the outside world. The Assassins themselves are divided into ranks, with the senior Lord Assassins no longer working directly but instead being responsible for planning, organising, research and watching over the younger Assassins. The greater number of the Assassins are unranked, though some can be more experienced than others.[2b] Under Imperial and Mechanicus law, the improper activation of a member of the Officio Assassinorum is considered treason of the highest order.
The temples use many exotic types of equipment, such as synskin, a substance that once sprayed onto the body, forms a black outer skin which protects the wearer from the environment and enhances strength.[2a] The Temple also maintains a secret vault of forbidden and alien weapons, giving them access to some of the most advanced technology in the Imperium.[5a]
The origins of the Officio Assassinorum are traced to the times of the Great Crusade where a pact was made on Mount Vengeance by the six Directors Primus of the Assassin Clades, headed by the Master of Assassins. All seven individuals were masked and their identities kept secret from one another and thus they were unaware that their leader was none other than Malcador the Sigilite. From the six Directors Primus, the founding Clades were formed from Sire Vindicare, Sire Eversor, Sire Culexus, Sire Vanus, Siresess Venenum and Siresess Callidus who all met in secret at the Shrouds. For decades, they hunted the enemies of the Emperor by way of stealth and subterfuge in order to show these foes that there was no place for them to hide.[3a] Such was the secrecy of this agency that its existence and actions were only marginally known to the Emperor as they intended to maintain his noble purity. As the deployment of an Imperial Assassin was a delicate matter, usually only a single agent was sent into the field or, at maximum, three, but always from the same primary Clade.[3c]
Following the outbreak of the Horus Heresy, the agents of the Officio Assassinorum dispatched eight agents from the six primary Clades in order to eliminate the renegade Warmaster Horus, but all of them failed. The last attempt was made by Tobeld of the Venenum Clade outside the command tent of the rogue Primarch. During a meeting at the Shrouds, the Clade Directors Primus debated and argued over how to eliminate their foe. At this point the Master of Assassins, following the advice of Legio Custodes Captain-General Constantin Valdor, decided to assemble the first Officio Assassinorum Execution Force consisting of various Clades, to eliminate Horus. Such an act was unheard of in the history of the Officio Assassinorum.[3b]
In 546.M32, the High Lords of Terra were slain to a man by the Grand Master of the Officio, Drakan Vangorich. A Space Marine strike force drawing from the Halo Brethren, Sable Swords, and Imperial Fists Chapters was scrambled to stop Vangorich. Storming the Assassinorum Temple on Terra, the force was assailed by a hundred Eversor Assassins. One sole Space Marine survived to reach Vangorich and slay the mad Grand Master. Afterwards, the Imperium descended into a period of anarchy.
During the Age of Apostasy, the Officio itself became afflicted by the general corruption of the Imperium, and certain high-ranking members of the Office utilised the assassins for Goge Vandire's own ends. During Vandire's purges, powerful individuals and even the Officio's Grand Master became victims of the assassins. The war within the Officio became known as the Wars of Vindication. Following the Apostasy, it was deemed wise to reorganise the Office, one change being that only the Senate could order an assassination. A section of the Inquisition, the Ordo Sicarius, was created to monitor and control the Officio Assassinorum from that date on.[Needs Citation]
It was through intelligence gathered by the Officio Assassinorum that the Adeptus Astartes learnt that the Sorceror Ygethmor intended to claim the world of Medusa V in a Warp Storm.[7b] During the 13th Black Crusade, an assassin team was deployed to eliminate the Sorceror amongst other notable targets. It was known that at least seven agents of the Assassinorum failed in the attempt on Ygethmor's life.[7a]
- M31 - The first Officio Assassinorum Execution Force is dispatched to kill the rogue Warmaster Horus. The operation ends in failure.11
- M31 - The Callidus Assassin M'Shen slays Night Lords Primarch Konrad Curze.
- 546.M32 - The High Lords of Terra are killed to a man on the orders of the rebellious Grand Master of Assassins Drakan Vangorich.
- 990.M32 - Culexus Assassin Dranos slays the Sorcerer Xantaka.
- 340.M33 - The Callidus Assassin Mother Gullet steals away the son of the dangerously self-centered Governor Thygmus van Spracht.
- 452.M34 - A Culexus Assassin assassinates the Eldar Farseer Lithandros-Esmanthil.
- 372.M35 - Six Eversor Assassins massacre the Abhuman Thugrock Goliath blood cult of Thugrock Secundus
- 501.M37 - Cardinal Jerome the Unsaintly is killed by a Vindicare Assassin after trying to secede from the Imperium
- 563.M37 - Venenum Temple Assassin Urhua Thereaux, originally targeting the renegade Governor Yawell Morisha, instead finds him dead and replaced by an anti-Imperial democratic committee. Thereaux kills every committee member over the next three days.
- 003.M38 - The progressive astronomer Lenas Scard is declared heretical and killed by the Vindicare Assassin Erasmus Menst.
- 386143.M38 - A Callidus Assassin manages to end the rebellion on Orlenza Triartes by replacing the rebellious lords chief military advisor
- 209.M38 - Callidus Assassin Militzia Scarvelli is able to impersonate a Gretchin, infiltrate the Ork mob of Big Mek Oilguzla, and assassinate him
- 243.M39 - Dark Eldar pirate pilot Skyknife is killed by the Vindicare Assassin Dejedris Garamach after the sniper waited in position for six years
- 353.M41 - The Culexus Assassin known as the Revoker kills a group of Rogue Psykers known as The Gestalt
- 400.M41 - A Callidus Assassin posing as an Inquisitorial Stormtrooper kills the already dying Warmaster and eventual Saint Lord Solar Macharius at the end of the Macharian Crusade. The assassination was part of a conspiracy involving the Inquisitor Drake, who was subsequently killed by the assassin as well to hide the truth of the matter. After the deaths of Macharius and Drake, Imperial authorities cover up the truth.
- ca. 809.M41 - A Vindicare Assassin fails to neutralize Cardinal-Astral Xaphan on Vraks Prime, precipitating several days of civil disorder and ultimately the Siege of Vraks.
- 886.M41 - An Eversor Assassin massacres the Tech-Priests of the lubricant-worshiping Lubricae Cult
- 999.M41 - All attempts to assassinate Abaddon the Despoiler amid the Thirteenth Black Crusade fail
Connections to Space Marine Chapters
Owing to their codes of conduct and their proud warrior nature, Space Marines find assassination distasteful, and will avoid any contact with assassins under most circumstances. However, as is to be expected from an institution which specialises in subterfuge and clandestine actions, there have been countless rumours and conspiracy theories concerning the Officio Assassinorum, and a number of these relate to disasters which have befallen certain Space Marine Chapters. A long string of coincidences have led some to lay the blame for the loss of the Fire Hawks' fleet in the warp, the unusual destruction of the Crimson Fists' fortress monastery by one of their own defensive missiles, the death of the Celestial Lions Apothecaries on Armageddon, and the ambush of the Angels of Retribution on Alantor X, as well as many other lesser known catastrophes squarely at the Assassins' door. It may never be known if Assassins were involved in any of these calamities, and if they were, whether they were working under the orders of the High Lords of Terra, some other institution or for their own agenda.[5b]
What is known, however, though it is hardly common knowledge, is that, since the Horus Heresy, some secret departments of the Inquisition have researched methods of controlling and/or killing Space Marines, and some Assassins have access to their work.
Agents of the Officio Assassinorum are killers and death machines, all of whom are trained in one of the hidden temples to either become an expert sniper, a biochemically frenzy-driven killer, a shapeshifter or worse. The known temples are:
- Vindicare Temple - sharpshooters, specialists in sniping and marksmanship.
- Callidus Temple - chameleons, specialists in infiltration and impersonation.
- Eversor Temple - berserkers, drug-fuelled killing machines.
- Culexus Temple - Pariahs, psykers are their exclusive targets.
- Venenum Temple - specialists in poisoning their targets.
- Vanus Temple - intelligence-gatherers, in matters of strategy and tactics their insight is unparalleled. They often assassinate their targets indirectly using their intelligence and knowledge to bring down targets. Fon Tariel, an infocyte of Clade Vanus during the Horus Heresy noted that: 'the cleanest kill is one that another performs in your stead with no knowledge of your incitement.'[3a]
- Maerorus Temple - Assassins developed using illegal human/mutant/xenos hybrids to create assassins capable of absorbing the biomass of victims to rapidly evolve new bio-weapons. 
The temples, with the exception of the Culexus Temple, are located on Terra but their specific locations are a closely guarded secret. They may be deployed to a battlefield to covertly 'help' an Imperial force, including Space Marines, Sisters of Battle, and the Imperial Guard, but never join any squad, as they work best alone.[Needs Citation]
- 1: Codex Imperialis, [Needs Citation]
- 2: Warhammer 40,000: Rogue Trader by Rick Priestley
- 3: Nemesis (Novel)
- 4: Inquisitor Rulebook Part Two, pg. 31
- 5: Codex: Assassins (3rd Edition)
- 6: Warhammer 40,000 5th Edition Rulebook, pg. 124
- 7: The Fall of Medusa V
- 8: Deus ex Mechanicus (Short Story) by Andy Chambers
- 9: Seventh Retribution (Novel) by Ben Counter, [Needs Citation]
- 10: Wolfblade (Novel) by William King - Space Wolf: The Second Omnibus, Chapter Twenty-Two, pgs. 214-215
- 11: Dataslate: Officio Assassinorum pg.28-33
- 12: Imperial Armour Volume Five - The Siege of Vraks - Part One pgs. 9-10.
- 13: Fall of Macharius (Novel), [Needs Citation]