So it's a time factor? Like attacks generated from Stealth automatically critical hit if completed within 3 seconds?
It's closer to 1 second I think. At point blank range, Hidden Blades seems to only benefit for the first 3 slashes. At mediumish range, I get 1-2. Trying to do it across the map, zero of them benefit.
Also, I don't think Hidden Blades benefits from flanking. I have like +50% flanking damage, don't see any difference in damage.
As for the poison attempt.... Now that I tried it, not worth it on Assassin.
1. Many things are immune to poison because reasons.
2. Deathblow actually needs the stealth autocrit to one shot Perilous archers at full HP reliably. Being that a major appeal of Deathblow over Twin Fangs is ability to Stealth-Kill-Stealth-Kill at no cooldown, no bueno. And there just aren't enough points to grab that if one goes for poison. Deathblow and Shadow Strike really need to switch places on the assassin skill tree.
3. The cloud poison itself is O.K. at best. Doesn't affect at how fast Deathblow tears down crowds at all. Mostly seems like it would benefit cooldowned large spread AoE attacks more than reaver-lite AoE of Deathblow.
4. The +25% dagger damage bonus upgrade seems bugged. I actually get reduced physical damage with poison on against many enemies. All my what. Not sure if UI bug or actual bug.
5. All in all, constantly buffing up the weapon gets irritating, even with +duration ring.
Side note: Poison seems to do increased damage on the first tick depending on the strength of the attack that inflicts it.