Each assassin may be taken as a one per detachment elite slot and comes under the armies of the Imperium for the purposes of allying. The faction is officio assassinorum (they now have their own tab on the GW page, although hilariously all the models are 'no longer available'). Operation assassinate is the command benefit and means you get an extra VP if the final wound on the enemies warlord is removed by your assassin(s). There is a second option in the form of a formation; the assasinorum execution force. This is effectively one of each of the 4 assassins and in addition to the above they get preferred enemy: warlord. A pretty fluffy way to field the assassins - so far so good.#
The assassins retain the same profile as before except they have gained an all important additional wound! All 4 assassins have fearless and move through cover as before, but now also have infiltrate as standard too. The other 3 common rules to the 4 are lightning reflexes (4++ and ignore difficult terrain penalty when charging), no escape (-2 to LOS rolls against wounds from assasins) and Independent operative (IC cannot join and assassin doesn't get warlord trait if your warlord). Synskin is now gone and fleet is no longer standard.
Now slightly more expensive, the vinidcare has some subtle tweaks. He retains blind (defensive) grenades and stealth, but has lost frag grenades. He also has a CCW now for that extra CC attack combined with his pistol. The exitus rifle (sniper) and pistol (pistol) have both changed to AP2 and the rifle has doubled in range! Hellfire shells are the same, shield breakers ignore ++ saves (for wounds and glancing/penetrating hits) now rather than removing them for the game and the turbo penetrator is s10 vs vehicles and D3 wounds vs anything else. The deadshot rule has also changed to bring it more in line with 7th - now all to hit rolls are precision shots (excluding snap shots). The spy mask also works for to hit rolls (excluding snap shots) and grants ignores cover.
She remains the same points wise and is a bit more deadly now I'd say. She retains fleet and hit+run but lost stealth. Importantly, she gained precision strikes as a general rule. Her polymorphine rule is now much more useful in keeping her alive as she can either infiltrate 1" away from the enemy or move on from the enemy table edge when arriving from reserve. In either case, the enemy can only snap shot her the turn of arrival. The reign of confusion rule means that you can reroll to seize the initiative and your opponent must -3 from their first reserve roll of the game. The neural shredder changed from being s8 against the targets Ld to being aways wounds on a 4+. It's still a template that can't harm vehicles or buildings but is now AP2 rather than AP1 (didn't make sense to be AP1 if it couldn't harm vehicles anyway). The Callidus retains her phase sword, which is now an AP2 melee weapon that ignores ++ saves on a to hit wound roll of 6. In addition, the Callidus now has poison blades as an alternative, which always wound on a 3+ and rend.
He went up in points slightly and is arguably still the weakest of the assassins but he got some rules improvements at least. He lost fleet but retained furious charge and feel no pain (now 5+) and finally got his old wargear back. He now packs both a power sword and neuro-gauntlet, with the latter being a melee weapon with fleshbane and shred. Frenzon now means that instead of D6 attacks when charging he gains a set 3 and (to compensate for losing fleet) can roll a total 3D6 for charge range. The executioner pistol is a bolt and needle pistol in one (s1 poisoned) and with his fast shot rule he can fire this 4 times each time he fires (note that it doesn't specify which phase). In addition, the sentinel array allows him to fire all overwatch shots at full BS. Lastly, Bio-meltdown returned and thus when he dies any model in D6" takes a S5 hit.
Quite a few changes here. Again slightly increased in cost and now has both fear and preferred enemy: psykers (watch out psyker warlords). The Culexus retained psyk-out grenades in their new Grey Knight style format but lost frag grenades and the psyocculum. Life drain is back! No armour saves in close combat and instant death to psykers (on a 6 to wound for everything else)! Etherium is now improved so that shooting and CC attacks against the culexus are BS1/WS1 automatically. Psychic abomination means that any psykers within 12" have -3Ld, don't generate warp charge and can only harness warp charge points on a 6 (sounds great for shutting down daemons). Psychic powers do not affect the culexus and any blessings/maledications in place cease to work if that unit is within 12" of it. Lastly, the animus speculum now has 18" range but retains S5 AP1. However, rather than being assault 2 plus all psykers within 12" it is now a psychic phase shooting attack (so you can still run/throw a grenade) which has as many shots as every ML within 12" plus up to 3 warp charge from your own pool. Great if you're in the enemy's face, less so if you're in amongst your own.
That's pretty much it. I've tried to highlight and summarise the changes as best possible but haven't gone into the wider implications, strategy etc as I'm sure more intuitive tacticians than myself will write such reviews in due course. What's clear to me though is that the Culexus has been very much overhauled for the better and I'm now tempted to buy one as a result. The Grey Knight release has been pretty controversial but, despite the cost involved and fact it's electronic, I think that rethinking and segregating the assassin codex was a good move. Cheers.