Why the Culexus Assassin over their brethren? Because of its abilities against the growing threat that is psykers. While our forces have the Adamantium Will special rule, which gives us a 5+ Deny the Witch save, it isn't nearly enough. Nor does it affect blessings. That is where the Culexus Assassin comes in. This model can destroy psyker armies by himself. But how does it accomplish that mission?
The first, and most important, special rule is Psychic Abomination. This rule is essentially a 12" bubble surrounding the miniature that lowers the leadership of all psykers (friend and foe) by 3. In addition, no warp charges can be generated from those units and they can only utilize charges on a roll of a 6. Awesome right? It would be if that was all it did. But alas, there is more! It also gives the assassin complete immunity to psychic powers. This is huge, as it greatly increases the durability of the assassin. That has to be it, right? NO! One more thing...! If the assassin moves within 12" of any unit affected by a psychic power (i.e. blessing, malediction), it ceases to exist. Invisibility? Gone! Doom? Gone! Prescience? Gone! This special rule is simply amazing.
What else do we have to play with? In terms of durability, we have a few things outside of Psychic Abomination. Etherium is like having your own personal permanent Invisibility spell that never ends, forcing everything targeting the assassin to resolve their hits at BS/WS 1. You also have the assassin standard Lightning Reflexes, which gives you a 4+ invulnerable save. We also have Fear/Fearless, though the former is a little redundant due to Etherium. It also has T4, 3 wounds, and I7.
Offensively, the Culexus Assassin is no slouch either. With WS8/BS8, Str 4, and 4 attacks...it has a sexy offensive statline. It has Preferred Enemy (Psykers), as well as the rule Life Drain. This prevents any armor saves from being taken from any close combat wounds caused by the assassin. In addition, on rolls of a 6 to wound, it causes instant death. Against psykers, all close combat wounds cause instant death. Combo that with No Escape (-2 to "Look out sir!) and you'll be happily killing characters by the droves. The main ranged weapon you'll be using is the Animus Speculum. This weapon has a unique "restriction" that powers its shots with psykers within 12", plus up to 3 warp charges from your pool (which we won't have as BT). Unfortunately, this weapon won't be much help against armies without psykers, as you won't have anyway to power it. However, if you are facing them, you can potentially have a large quantity of Str 5 AP1 shots into your opponent. This can be devastating to your opponent. There is also Psyk-Out grenades for funsies.
With the inherent durability of the assassin, plus Infiltrate and Move Through Cover, this unit can be where-ever you want it and can live long enough to do the damage it needs to. All in a 140 point package. I don't know about you, but with as many psykers that are out there nowadays, this will be in EVERY army I make from now on. If you combine the assassin with a Techmarine's (i.e. Thunderfire Cannon) Bolster Defenses, you'll also get a 3+ cover save to go with your 4+ invulnerable save. ;)
Thoughts? Think there is a better place to put your 140 points?