"Ask not for whom they seek, lest it be thyself."
- – Tactica Imperium on The Officio Assassinorum
"The best government is a benevolent tyranny tempered by an occasional assassination."
- – Voltaire
"Nothing is true, everything is HERESY!"
- – A slightly redacted motto of the Officio Assassinorum's precursors from ancient Terra
You can tell you don't mess with the Officio Assassinorium. Anyone, from the lowliest of workers to the High Lords of Terra, at ANY moment, is open to either having their brains spilled on pavement by a sniper round, their spleen torn out by screaming Wry-man, or shanked by dear old mum for no apparent reason. When the shit hits the fan, these are the guys that get their hands dirty. It's all about the wetwork. This branch of the Administratum recruits, trains and commands Assassins of all kinds; for more information on the individual Assassin types, please visit this page.
The Officio Assassinorum was founded at the beginning of the Great Crusade as a department for holding any up-and-coming warlords or heretical leaders accountable for their actions. And by "accountable" we mean "disposed of". And by "disposed of" we mean "killed so hard that they will die to death". Originally such work was performed by the Alpha Legion, although like much of those bastards' history it's hard to be sure. Formed by Malcador the Sigillite and the six masters of the Clades (the future temples), it was founded in extreme secrecy in an attempt to keep it out of the Emperor's knowledge and maintain his purity (a concept the Big.E always found funny). It's even more amusing that they thought they could hide something from someone capable of basically hearing the thoughts of every human at once. And they all seem to know of one another.
In the beginning, the Officio acted independently in the shadows under the guidance of Malcador the Sigillite, their existence kept mostly secret. As mentioned, Empers knew from the beginning but tolerated the Officio. Constantin Valdor and the guys in gold found out pretty quickly there were other people dealing with threats to the Emperor behind their (and His) backs, but they too decided the Officio was a-okay. This unwritten arrangement worked out relatively well in the beginning until a large quantity of fecal matter hit the air circulation device. Malcador first sent individual assassins then a whole execution force after Horus, but the assassins were unable to kill the wayward Primarch. Worse, Rogal Dorn learned about the existence of the Officio. Being less... tolerant... than the Custodes, Dorn did not like what he saw. So he did the sensible thing and invited himself to one of the Officio's meetings, intent on voicing his opinion on their modus operandi tactfully. Malcador, who would not take shit from Dorn, politely but firmly told him he should grow up. Luckily, at that point Big.E himself walked in, told Dorn the existence of the Officio was just as planned and that he should leave them alone, but at the same time he told Malcador to drop the cloak and dagger and that the Officio from now would officially be recognized and work under strict rules of engagement.
From that time on, a terminatus order could only be issued by the Senatorum Imperialus (AKA the High Lords of Terra) and the members of the Officio Assassinorum are kept on a very short leash. Unlike the Inquisition, they have no discretionary powers of their own, because they tend to horribly abuse them if they do.
The head of the Officio is known as the Grand Master of the Officio Assassinorum, or The Master of Assassins, who sits alongside the Fabricator-General of Mars and the Ecclesiarch of the Adeptus Ministorum as one of the High Lords of Terra.
Which, if you think about it, must be somewhat distracting during their meetings. The Lords are sitting around and discussing the fates of trillions of men, and as the debate goes back and forth, one of them says nothing, ever. He just sits there, smoking a lho-stick with his trench coat wrapped around him, almost completely overlooked. And then they remember that a bad decision could get them killed, and this is the guy who does that. And only that. His only job is to dispatch death on swift wings to any high official. It'd be like having the Sword of Damocles hanging over your head everywhere you went.
Members of the Officio are recruited at a very young age; most of them are children from Feral Worlds or are taken from the Schola Progenium. Recruits are biologically augmented and trained at the School of Assassins on Holy Terra for ten years before being assigned to one of the Assassin Temples to continue their training and eventually serve the Imperium.
The Assassin specialists approach Sly Marbo levels of awesome, as the Temples have permission make use of "any means or tools to get the job done," even if it means banned tech from the Dark Age of Technology (like the Terminus gland) or xeno artifacts (like the C'tan phase blade). These Assassins are also given crushingly restrictive psych treatments to ensure they never use these tools except when ordered, which can make them seem kinda spastic during downtimes when said tools are augmentations to their own body.
There are only 2 recorded examples of anyone in the Officio turning against the Imperium. Unfortunately, both times it was the Grand Master, Drakan Vangorich. The Imperium learned of the first one's butthole treachery when he had the High Lords of Terra wiped out to the last man. Very grimdark, though the uncaring dicks had it coming to them. It could be said that Vangorich getting rid of the incompetent idiots was a good thing for the Imperium, but he did more harm than good in the longer run with his increasingly totalitarian (totalitarian compared to the 'regular' Imperium standards which are already pretty fucked up, mind) ways to prevent the situation from going to shit ever again.
After that betrayal, an Astartes retribution force was dispatched to "resolve" the situation. It is not known how many marines were sent; since the force was made up of elements of three chapters (Halo Brethren, Sable Swords and Imperial Fists), it was probably at least 1000 marines (400 Marines landed, half survived the initial purging, just one survived the whole “fight the Assassinorum” thing which at least proved that even ambushed by hundreds of assassins, the outnumbered Astartes were still superior). As soon as they landed on Terra, the commander of the force was assassinated. The remaining marines broke into the grand chamber of the Assassinorum Temple and found 100 (!) Eversor Assassins between them and their target. A single marine survived to plug the Grand Master in the brain with a boltgun. This super-hardcore-awesome-ass-kicking marine was definitely as badass as Darnath Lysander. As it turns out, it was the Imperial Fists' Chapter Master!
The other one was Vindicare Assassin LIIVI.
Ha, just kidding. The second time was during the Age of Apostasy in M36, when the Grand Master was himself assassinated by a Callidus Assassin under the orders of Goge Vandire, the insane ruler of the Imperium. Of course, you don't become Grand Master without being an Assassin yourself, and it turned out that he had switched places with a body double. What followed was a civil war within the Assasinorum to mirror the one wracking the Imperium, and ended up filling the Imperial Palace with nerve gas and destroying entire wings of Mankind's greatest fortress with ancient Dark Age and xenos weaponry. Eventually, the Grand Master triumphed, but went into self-imposed exile at the end of it, and might still be alive today. After the war, the Inquisition set up the Ordo Sicarius to monitor the Assassins and make sure they stay in line.
Alas, they will always be second best, for they can never become Sly Marbo . They only exist in fluff for the purposes of showing just how bad-ass a specific enemy is. For example, during the Horus Heresy, eight(!) Assassins, at least one from each temple, was sent to form an Assassin's Justice League of Doom and kill that asshole who started it. For SOME REASON, like everyone else in the Imperium who tried to stop it, they failed. To be fair, their mission was impossible anyway. The Beheading’s ultimate end showed even normal Astartes are far superior even when ambushed by numerous Assassins of many types. A mere eight assassins against any Primarch is just providing him with a moment’s entertainment. Also, the same thing was tried during the Medusa V campaign, to assassinate the leader of the Chaos faction; seven of them failed, getting a front row seat to some Eldar stealing their kill. Imperial Armour books goes even further and currently have a Vindicare torn apart by an angry mob of Vraksian heretics after failing to shoot down his target, and Eversor being ripped to shreds by point-blank pulse carbine fire and only taking down one of his targets due to a tactical nuke strapped to his chest, with both cases causing the completely opposite effect imperials hoped for. This is known as the "Worf Effect" by Star Trek fans, or "The Avatar of Khaine effect" by Eldar players.
The Officio Assassinorum has six "temples," or specialist branches, each focused on one particular method of assassination:
- Callidus - shapeshifters, masters of infiltration. Almost all women (or women pretending to be Orks, or...just...try not to think about it).
- Culexus - soulless gothtards that are invisible & immune to psykers, daemons & the Warp.
- Eversor - cyborg, drugged-up killing machines that have two settings: coma and RIP'N'TEAR!!1!!1!one!
- Vanus - leet hacker NERD CHESS PLAYERS. All are genii in the fields of intelligence gathering, strategy and JUST AS PLANNED. "The cleanest kill is one that another performs in your stead with no knowledge of your incitement." They are what every neckbeard wants to be.
- Venenum - master poisoners who can take the things found in a vegan's kitchen and kill a Space Marine with the resulting mixture (death by malnutrition, presumably).
- Vindicare - snipers that can "BOOM! HEADSHOT!" from beyond the horizon.
- Maerorus - a failed attempt at creating a living weapon able to operate without support or equipment for a prolonged period of time. The only one created proved to be impossible to control and went rogue; all attempts at making more were immediately discontinued. Think Alex Mercer from Prototype, only even more sociopathic.
Death Cult Assassins aren't part of this framework; there are thousands of death cults throughout the Imperium and they each train in their own way to kill enemies of the Emperor, but are not officially-sanctioned and are more autonomous than the Assassins.
Assassinorum: Execution Force
GeeDubs released a (rather awesome) Space Hulk-style board game for 1-4 players that revolves around an Execution Force - an Officio Assassinorum team comprised of a Callidus, Culexus, Vindicare, and Eversor assassin. Gameplay is a combination of Splinter Cell and X-COM, wherein the four player-controlled Assassins run through the map attempting to find a teleportarium room, which in turn will allow them to move to the second (and final) map. Once there, players must kill a boss of sorts - a Chaos Sorcerer who is trying to blah blah kill the Imperium with blibbity bloobity Chaos shenanigans. You've heard the story before. Along the way, Assassins fight Renegades (Chaos Cultists and Spess Muhreens) which are spawned either by revealing rooms (the only way to find the teleportato and its control room) or through random events. Renegades can be alerted to the Assassins' presence by catching them in line of sight or by hearing the sounds of glorious gunplay down the hall. The Assassins, of course, are all badasses with unique attributes and abilities, including Primaris and Omegon Tactics (super special powers that can be used thrice or once per game, respectively). All in all the game is an overcosted and short-lived boardgame compared to Space Hulk, which for the same price gave you almost triple the amount of good guys and also more uniquely posed bad guys as well as over a dozen different missions with differing map layouts. Not to mention the map tiles were embossed as well. This means you would need a week or so of daily playing to finish all Space Hulk missions, while Officio Assassinorum will get boring after 3 or 4 plays. A shame, GeeDubs. A shame.
On the upside, you can take those four assassins and bring them over to a standard 40k game as henchmen for an Inquisitor.
Eversors are the That Guy of Assassins.