Nice build. I like the specialist ones they turn out to be more of a challenge and more rewarding when you come out at the other end (beat Legendary) so to speak.
I can see your dilema with the poison damage to undead. One of my other characters spirit/warfare - centred around leeching life and damage and found it a struggle against undead and constructs due to their high resistance to draining attacks - however if I were you just persist and maybe look at a secondary damage type through the rogue tree or a weapon switch that does a lot of damage in another area.
I still think it is cool going the extreme poison idea, like I said before it makes the game more of a challenge, gives your character a wee bit more uniqueness to him (rather than going a build that is tried and true) and is more rewarding when you overcome a Telkine or Big boss of some type.
Good luck.
