How to deal with grey knights and blood angels

Honestly, as with Draigo and paladins, it's just not a unit I'd even bother trying to kill until it's literally the only thing he has left in his army.

I'm playing a Grey Knight army tomorrow. I have a Culexus sitting around, so I'm gonna bring him out to play. Do people think he'll be a useful unit?
I plan to infiltrate him, first turn =boost up in raiders to support/distract and hopefully get Culexus into combat T2. Where he should murder everyone.

The Culexus will cause the Draigostar to have kittens. Only drawback is that if you get into assault Draigo will probably challenge you as he's least affected by the Culexus's abilities (ignores ID and has a 3++ save). Also, if there's nothing else in the same combat the whole unit will still attack you whilst Draigo tanks your attacks so make sure there's at least one other unit in the fight. Stay out of combat as long as possible and use the Animus Speculum to take down as many of the unit as possible, making sure to attack from an angle where Draigo is not the closest model.

Not sure if he actually uses Draigo and/or 'Draigostar". Will keep the Challange/3++/Position of Draigo thing in mind for CC.
I dont have a GK codex, does every GK model have the Psyker special rule attached? Just want to make sure Life Drain will work on everything.

Everything except non-walker vehicles.

So everything that the Culexus Hits in Assault (not wounds) excluding non-walker vehicles, will suffer Instant Death. That's awesome.

ID only kicks in on unsaved wounds but essentially, yes. That's why Draigo can stand up to the Culexus - he's Eternal Warrior.