Now that Assassins are no longer in the Grey Knights Codex (that review coming soon), they instead are included in their own dataslate, opening them up to be taken in just about any army.
The Dataslate begins with some quick fluff on the four main combat assassin temples, followed by the Officio Assassinorum Detachment, which is simply a single Elites choice. This must be an Assassin, who gains the Operation Assassinate rule that if Victory Points are being used, the player gets an additional victory point if the Assassin removes the final wound from the opposing warlord.
Next is the Assassinorum Execution Force Formation, which includes one of each assassin. The benefit of the formation is that the Assassins gain Preferred Enemy (Warlord), as well as the Operation Assassinate rule.
First up is the Vindicare Assassin, checking in at 5 points higher than before, with an extra wound. All shots, outside of snap shots, are Precision Shots. He cannot be joined by other characters. He has a 4++ save and ignores the penalty for charging through cover. Enemy characters suffer -2 to their Look Out, Sir rolls for shots made by the Vindicare. His Exitus Rifle and Pistol are AP2, and have an option of three rounds - Sheild Breaker (Ignores Invulnerable saves), Turbo-penetrator (S10 vs Vehicles, deals D3 wounds vs. other targets), and Hellfire (wound on a 2+). Finally, his Spy Mask gives all shots (except snap shots) Ignores Cover.
Next is the Callidus Assassin, at the same point cost, but again gaining a wound. The Callidus cannot be joined by characters and will never get a Warlord trait. She also gets the Lightning Reflexes and No Escape rules (4++, no Initiative Penalty, and -2 to LO,S tests). Additionally, she gets the Polymorphine rule, allowing her to deploy anywhere more than an inch from an opponent, or if held in reserves, enter from the enemy board edge. In the first turn she is on the board, only Snap Shots can be fired at her. Her other special rule allows the controlling player to reroll the seize roll and gives a -3 penalty to the opponent's first reserve roll. Her Neural Shredder is a template weapon that wounds on a 4+ and has AP2. The Phase Sword ignores invulnerable saves on a 6 to wound, and is AP2. Finally, the Callidus also has Poison blades, which are Poisoned (3+) and Rending.
The Eversor is 5 points more expensive, but again, gains a wound. The Eversor has the standard Independent Operative, Lightning Reflexes, and No Escape Assassin Rules. In addition, he has the Bio-meltdown (all models in D6" take a S5 hit when the assassin is killed) and Fast Shot (can fire his Executioner pistol 4 times at his target, in any combination of modes). The Executioner pistol can be fired as either a needle pistol (poisoned) or a bolt pistol. His Frenzon combat drugs allow him to charge 3D6", and get 3 Attacks for charging instead of 1. The Eversor fires overwatch at his full BS. Finally, he has a power sword and neuro gauntlet, which has fleshbane and shred.
Finally is the Culexus Assassin, who is also 5 points more expensive. It comes with the three standard Assassin rules, as well as Life Drain and Psychic Abomination. Life Drain allows its close combat attacks to ignore armor and cause instand death on a 6 to Wound. All attacks on psyker have the instant death rule. The Psychic Abomination rule causes psykers within 12" to suffer -3 Ld penalty, not generate Warp Charge, and only successfully harness WC points on a 6. The Culexis can't be the target of psychic powers, or affected by them. Finally, all blessings and maledictions are nullified if a Culexis is within 12". The Culexis's wargear is fairly complex. The Animus Speculum is fired in the psychic phase with a number of shots equal to the dice nullified by the assassin. All shooting and close combat attacks against the Assassin are resolved at WS/BS 1. Finally, the Culexis carries Psyk-out grenades.
For me, the real standout is the Culexis, who is a major game changer against all opponents who rely on psychic powers. It also has fairly strong defensive rules to keep it alive all on its own.