The Inquisition is able to call upon a single operative of the Officio Assassinorum to help assault the Daemon and heretic. These bio-modified humans are trained under a certain temple, each with a specific goal in mind. The Vindicare to slay with marksmanship; the Culexus to kill the psyker; the Callidus to infiltrate and assassinate; the Eversor to wreak havoc in close-combat. All of them are ruthless and cunning killers, yet each with their own flavor of death.
The Officio Assassinorum Operatives all have some distinct special rules shared, but for the most part majority of their rules are specific to their temple. All assassins are Fearless, have the infiltrate special rule, and have a 4+ invulnerable save. In addition, they all have above average Weapon Skill and Ballistic, and Strength and Toughness of 4. Now, onto the specific temples.

• The Vindicare Assassin - Specializing in sniping, the Vindicare assassin carries an Exitus Rifle, a sniper rifle with an AP of 2. In addition, the Vindicare assassin has four special rounds, one of each, that can be declared used before you roll to hit. Shield-braker ignores any invulnerable save. Turbo-Penetrator inflicts 2 wounds on any model wounded and has an Armor Penetration roll of 3D6 against vehicles. Hellfire rounds wound on a 2+. The Vindicare Assassin also carries a powerful pistol and has a mask that reduces his opponents cover save by -1 (a 5+ becomes a 6+, etc.) and rolls 2D6x5 for Night Fighting. His Stealth Suit also makes it so he is affected by Night fighting. The best part about this assassin is that the shooter nominates the model who takes the wound in a squad and can target any model in range and in line of sight, regardless of any targeting restrictions (such as units in close combat).
  
  The Vindicare Assassin is quite useful. He has a chance to penetrate any vehicle in the game and can take out Sergeant with a Power Fist, allowing your Daemonhost to charge without worry. The Assassin is also great at picking out powerful characters in retinues or special weapons in squads. The problem is his ammo can only be used once per game, and he will only get one shot a turn, at most taking out one model. But those models can be crucial ones and you can gain a huge board advantage by taking out units your opponent needs the most. Deploy him in cover in a building or at a high point and be sure to know your targets before the game even starts. Maybe you have an open eye for that Rune Priest or maybe you know that this Defiler is going to give you trouble and you want to see if you can get that lucky Turbo-Penetrator shot off. Where you place him depends who he will be able to shoot. He has a 36" range, not an unlimited one. He is great with Daemonhosts and in any Daemonhunter army, really. However, I usually take a Callidus Assassin, but this guy is tied with her for best assassin.
  
  
• The Culexus Assassin: This assassin's goal is to find and take out assassins. Being Soulles, the assassin lowers all units with a model within 12" Leadership to 7, unless it would normally be lower. This helps out as is for killing psykers and negating their power. However, there is much more. The Culexus Assassin has a 12" Strength 5, AP 1, Assault 2 weapon that gets another shot for every psyker within 12" (including friendly ones, ie. Justicars, Brother-captains, and Inquisitors). In addition, the assassin may throw a Psyk-out grenade at psykers. They have a 6" range and forces the psyker hit to pass a Leadership test on a 2D6. For each point his opponent fails by, he loses 1 Wound. Saving Throws may be taken as normal. Any psykers within 6" of this assassin must also take a Morale Check at the start of their turn and will fall back (and so will their squad) if they fail. All of this added with the Culexus Assassin's ability to specifically target psykers with shooting attacks, regardless of whether or not the psyker is part of a unit. The Culexus Assassin may also ignore other models when charging into an assault if this would allow contact with a psyker. If in close combat with a psyker, before any attack are made, both players roll 2D6 and add their Leadership value. If the Culexus Assassin wins, the psyker loses 1 Wound with no saves allowed. This happens at the start of every close combat round and affects a maximum of one psyker.
  
  Psychic meta-game, meet your anti-unit. The Culexus Assassin can pull some insane damage on opponents if they take a psyker or two. The assassins shots are boosted by nearby psykers and being AP 1 helps (Strength 5 can pen rear armor of vehicles) and he can take out psykers easily in close-combat. You have to be careful of hurting your own psychic powers/hood with his Soulless ability, though, even if most of your units are Fearless. Because of this, using Infiltrate/outflank allows you to appear out of seamingly no where and ruin your opponents psykers day. Effective against anything from the Daemon Prince, the Psyker Battle Squad, Tyranids, and Farseers with Warlocks, this assassin ruins your opponents day. However, if you were using this guy and your opponent didn't even take one psyker, he isn't too great and you would have paid an awful lot of points for a worse Vindicare Assassin. Still, chances are your opponent will take one psyker is high and you can easily trade point for point against them. Some things to be aware of are that Psyk-Out Grenades are... nearly useless. You are almost always better off using the Animus Speculum. After you took out your opponents psyker, this guy isn't useless either. His AP 1 shots are always great, and he can hold his own in close combat, making even Marines fail morale checks. Just don't over extend this guy to take out the psyker first turn; he doesn't have any bonus defenses.
  
  
• The Callidus - Trained in shapeshifting and espionage, the Callidus Assasin slays foes and takes their armor and their form, using Polymorphine to take on a new humanoid shape. The Callidus Assassin has a C'tan Phase Sword, too, allowing her to take out the toughest of opponents as it allows no saves whatsoever! Her Neural Shredder uses a strength of 8 against your opponents Leadership value with a flamer template and an AP of 1. She also gets one attack after all combats have been resolved, wounding on a 4+ with armor saves allowed. The Callidus Assassin can also Jump Back at the start of an Assault Phase (before charges are declared). Roll a D6, and move that many inches away from the enemy unless you roll a 1, at which point you do not move. If the enemy is unengaged at the end of the assault phase, they may consolidate. What makes the Callidus so great is she must start in reserve, but upon coming out is placed anywhere, and can move and assault as normal. In addition, at the start of the game, before the first turn, you may move any model 6" as the assassin disguises herself as an enemy and influences your opponents strategy.
  
  The Callidus Assassin can be considered the greatest in the book. She can come in any turn and assault any enemy, not allowing them to take any saves. Plus, A Word in Your Ear is possibly the greatest pre-game ability in the game (if that makes sense). Use it to force dangerous terrain tests on vehicles (once as you move them into terrain and again when they leave), move heavy weapon squads around out of Line of Sight, force that barrage or long range weapon out in the open and up-close, or pull units away from the transport they were hoping to jump into Turn 1. The Neural Shredder also is quite strong, being AP 1 can allow you to kill any mediocre leadership squad and even superglance (-4 on the damage table, but -3 for being AP 1) any vehicle. You should also be aware that you use the squads majority Leadership value, not the highest, as the Leadership of the squad is used as the Toughness. (Open to interpretation, however) Also, a word of wisdom is to make sure you can go first. You won't do much against that initiative 6 HQ if he kills you before you can even attack, and make sure you don't charge through terrain. There is also confusion on whether you Jump Back before or after charges, so when you use it is completely different. If you do it before charges, Jump Back on your turn and recharge your opponent for the extra attack. Overall, the Callidus is a great close combat machine, taking apart squishy squads and Independent Characters alike. Just be sure you use A Word in Your Ear, it is quite easily forgotten but simply amazing. (I recommend holding the Assassin as you deploy to remind yourself.)
  
  
• The Eversor Assassin - The Eversor Assassin is a master of poisoning weapons, drugs, and close combat. Armed with a bolt pistol/needle pistol combo, the assassin can fire a shot that wounds on a 4+ with an AP of 6 or a strength 4 AP 5 shot. The pistol is not where the Eversor shines, however. The Eversor Assassin is equipped with a Power Weapon and Melta-bombs, but also a close combat weapon called the Neuro-Gauntlet. The Neuro-Gauntlet rolls to hit as normal but wounds on a 4+ and disallows armor saves. Vehicles take a glancing hit on a 6, regardless of armor value. To increase combat potency, the Eversor Assassin takes combat drugs that allows him to charge 12" and he gains +D6 attacks on the charge instead of the normal +1. When the Eversor Assassin falls in battle, he does not go without a fight. If he is killed, place a blast marker with the center over the model. Any model touched by this takes a strength 5 hit, with saving throws as normal.
  
  The Eversor Assassin is one I have used many times. Throwing down up to 10 power weapon attacks on the charge first turn from an infiltration location is quite scary for most opponents. Plus, if they kill him, he will only take more opponents with him. He is also the only Assassin with two close combat weapons, giving him 4 attacks base and he has melta-bombs, allowing him to charge a Land Raider and kill it first turn with an automatic hit. The Eversor assassin is also great for tying up enemy units. Barely touch that guy at the end of the squad and try to take as few hits as possible while landing handful of blows on your opponent but then slowing his attacks after that. The Eversor isn't a huge killing force by himself, but he works really well with Death-cult Assassins and other assaulting units, including a Grey Knight squad, and can be your one key at killing Wraithlord and Carnifexes. I personally rarely use him now, opting for the Callidus in most situations, but he is by no means weak. He can put out many more wounds than the Callidus and even get turn one assaults out, but he doesn't have the same A Word in Your Ear versatility that the Callidus does. Just be sure to back up this guy. He is great on the charge, but begins to die down after that, losing his D6 attacks, so make sure you reinforce him. Be aware that against some toughness models (T3, so Eldar, Dark Eldar, Tau, Imperial Guard, and so on) his power weapon will cause more wounds than the Gauntlet, so pick your weapon wisely and remember this applies to his pistol, also. His Bio-meltdown will also make your opponent cringe at his decision to counter-assault the assassin, so make sure you don't forget about that. Just don't let him blow up in the middle of your force, hitting an allied unit or two in the explosion.

Well, there you have it. The conclusion to the Elites choices. These Assassins are great, but remember you need an Inquisitor to take one of them (and you may only take one, sorry). Be sure to pick an assassin that fits your army correctly, and remember that placement is a huge deal with these guys. If you do infiltrate them, make sure you either have the view range you were looking for or the assault range you wanted. And don't forget about those odd things, such as the Vindicare's Exitus Pistol (which can shoot into close-combat and pick models, too), the Culexus' Animus Speculum being boosted by your Justicars, Brother-captains, Grandmasters, Inquisitors, and even Grey Knight Terminators with Holocaust, A Word in Your Ear from the Callidus Assassin, and the "poisoned" vs non-poisoned attacks on the Eversor. Let your enemies fear what hides in the shadow of the hammer.