For the D&D class/archetype, see Rogue.
Assassins are highly trained agents who are dispatched to kill a specific, high value target(s), usually using stealth and cunning. Many have their own methods of dealing with their target, such as using poisons, blades, or ranged weapons, or simply brutal cunning, though they all usually involve being very discreet and virtually undetectable or traceable, allowing them to escape once the job is done as if they were never there.
The word Assassin comes from Hashashin, an unorthodox Shia sect of Islam that existed between the 11th and 13th centuries which focused on publicly murdering political and religious rivals.
We also keep saying usually because there's a specific breed of assassins who likes to instead use the stealth of a Baneblade and the cunning of a Carnifex, all the while having the subtlety of an Khornate Berzerker, to kill their targets (There are no parenthesis on this pluralization because said assassins will murder their way to their target, instead of sneaking then murdering).
Imperial assassins are operatives who function under the command of the Officio Assassinorum, a branch of the Imperium. They are usually 'recruited' (kidnapped) at a young age, generally from orphans, and occasionally certain physiological properties are required (i.e. psychic blanks needed for the Culexus temple). The Assassinorum's relentless training and biological augmentation of these children through their lives turns them into the perfect weapons. By the time they are ready for active duty, they are as strong and tough as a Space Marine, faster than an Eldar Banshee, more deadly in close combat than a Grey Knight Grand Master, and more accurate at range than just about everything.
Imperial Assassins will be from one of the six 'Temples' of the Officio Assassinorum:
- The Callidus Temple has assassins that allow them to shape shift, allowing them to change their appearance to help them get close to their target.
- The Culexus Temple has assassins that are psychic blanks, which are the ultimate anathema to psykers. Thus, they exclusively deal with psychic targets.
- The Vindicare Temple has assassins that are some of the ultimate marksmen in the Imperium, allowing them to take out targets from kilometers away.
- The Eversor Temple has assassins that are about the equivalent of a portable genocide campaign. To describe their methods in a nutshell, they're about the equivalent of unleashing a World Eaters warband against a Slaaneshi pleasure cult.
- The Venenum Temple has assassins that specialize in poisons to kill their targets. They are however, purely infiltrators, so they don't see field combat use.
- The Vanus Temple, who emphasize gathering information on you before ensuring someone else kills you with so much deniability that even they don't know an assassin was involved.
- The Maerorus Temple, a failed attempt to create the perfect killing machine. Only one Maerorus was created, and the temple was disbanded when she proved to be uncontrollable.
The last three are less well known about; they don't have models since they're largely 'behind the scenes' killers and don't really fit into tabletop 40k, while the third...didn't work the way the Officio Assassinorum had in mind.
All assassins are equipped with 'synskin', a second skin that is sprayed on before a mission, which hardens and forms a perfectly fitting suit which dissipates the kinetic energy from incoming projectiles, allowing them to survive shots that could have killed them otherwise, and they possess such incredible reflexes that they can literally dodge incoming fire.
The four main temples chiefly differ in equipment and tactics:
- The Callidus temple uses the art of camouflage and subterfuge to achieve their goals. Their operatives use a drug called 'Polymorphine' to actually change the shape of their bodies. This, combined with impeccable knowledge of the cultures and mannerisms of the many species of the galaxy, allows them to believably impersonate anything, from a hulking, brutish Ork, to a beautiful human woman. Due to the nature of their work, the Callidus tend to be the most sociably able of the four assassins. They are frequently equipped with a C'tan phase blade, which can slice through Adamantium like butter, and causes instant death to any unfortunate enough to feel its edge. This is complemented by the ranged fury of the 'Neural Shredder', a small pistol that affects a large conical area in front of the barrel. Rather than damage foes depending on their physical toughness, it affects them differently depending on how mentally resolute they are, cowardly foes will fall like chaff to this weapon, whereas courageous enemies can shrug off its effects. Callidus are also significant psychological tools, if an enemy knows that any one of his 'allies' right next to him could be a Callidus, who knows how he may react?
- The Culexus temple recruits solely from psychic 'blanks'. Blanks are beings that have no presence in the Warp (essentially soulless). Blanks cause agony to nearby psykers, and can even kill weaker ones at close range. Due to this Culexus assassins are generally deployed when the oppositions force features many psykers (i.e. Eldar). The presence of a blank chills the soul of nearby non-psykers, and as such, they are generally avoided by all that don't absolutely have to be in contact with them. Culexus assassins are equipped with the 'Animus Speculum', a skull shaped helm that can concentrate the operatives anti-warp signature and launch bolts of energy across the battlefield, which shred right through flesh and armour like paper. When allied psykers are near, the Culexus's anti-warp energy increases, allowing him to launch more bolts from his Animus Speculum. They also possess the 'Etherium', a device that
lets you find a flying castleallows the Culexus to step out of phase with the material world, making the opposition question what they're aiming at is really there; only the most resolute foes dare to fire at the mysterious shadow of despair stalking the battlefield. The Culexus assassins are also equipped with a 'Psyocculum', a device which allows them to see the flow of the warp overlapping with this reality; through this vision, psykers shine like a fire, allowing the Culexus to easily find and impeccably target enemy psykers.
- The Eversor temple firmly believes that the nuke is mightier than the sword. Rather than quickly taking out a few key targets, the Eversor annihilates vast numbers of enemies in a whirlwind of gore. When sent into battle, an Eversor is pumped full of 'Frenzon', an extremely powerful aggression raising drug, that allows the operative to unleash a flurry of blows before the enemy even knows they are under attack. Outside of combat Eversors are heavily tranquilized to the point where they are essentially in an induced coma. This means they are human only in classification, their minds filled with nothing but desire to destroy the enemies of the Emperor. Eversor assassins are equipped with a 'Neuro Gauntlet', a vicious weapon that consists of 5 small neuro-toxin injectors mounted on the Eversor's fingers, as well as a Power Sword. For ranged combat they have the 'Executioner Pistol', a modified bolt pistol that fires bolt rounds that can tear through any flesh and armour with frightening ease, and alternately highly penetrating, envenomed needles. In case they encounter something too big to simply rip to pieces, they possess Melta-bombs to ravage enemy Armour. Also, killing one will cause his "Terminus gland" to trigger, turning his blood into a highly caustic, combustable substance and then striking a match, resulting in a huge explosion of armor-melting super-heated acid.
- The Vindicare temple specialises in long-ranged combat, with their operatives being the greatest snipers in the Imperium, and probably the entire Galaxy. Vindicare training concentrates on patience (as the assassin may have to remain still for days on end to attain the perfect shot), and emotional repression, lest the Vindicare find himself unable to pull the trigger at the vital moment. (In real life, sniping is considered an almost perversely personal profession for exactly that reason.) They have also been taught to focus their senses, allowing them to pick out a single target no matter how many allies it may be surrounded by. Vindicares are equipped with the finest sniping weapon known to man, the Exitus Rifle, a veritable masterpiece of firearm, powerful enough to tear through Tactical Dreadnought Armour, and accurate enough to hit a Bloodthirster in the eye. This is paired with the Exitus pistol, smaller and less unwieldy, but no less deadly than the rifle. They also have access to special ammunition, such as bio-acid Hellfire rounds that eat through living tissue, shield-breaker rounds that permanently disrupt force fields, and anti-vehicular rounds that can punch right through a Land Raider. They also possess 'Blind' grenades, grenades that explode with a brilliant flash of light and infra-red radiation (so heat-vision won't work), that dazzle assaulting foes and ensure they can't catch the Vindicare off guard.
- The Death Cult Assassins may be considered Imperial Assassins, even though they are not under the Officio Assassinorum, many inquisitors employ them for their skill with the twin power weapons they carry. They are able to strike several deadly blows on a foe very quickly. Whilst still dangerous foes, they in no way compare to 'official' Assassins.
The other guy
The entire concept of the Assassins evolved from a single entry in the original Second Edition army list for an Imperial Assassin, who could take any combination of wargear and a special card exclusive to him, Polymorphine, which allowed him to disguise himself as another model. This duly proved completely broken since there was no specific rule to say a Gretchin couldn't be an Imperial Assassin wearing Terminator Armour and riding a bike, and abuses of the Polymorphine Wargear card (especially those involving Vortex Grenades) seem to have been a large part of the reason why the later version of it made it exclusive to the far more limited Callidus Assassin.
TL:DR: They evolved from cheating.