I'm becoming a huge fan of the swamp spirit.
One thing I will mention though, when you first get poison cloud it has a Rest of 4, and the cloud lasts 4 turns (90% sure of this, but I don't have a save to check).
After an upgrade the Rest is reduced to 3, but the cloud now only lasts 3 turns.
I'm also a bit of a fan of Poison Spit.
At lower levels I heavily used the Sword spirit, but now the enemy stacks are too large for physical damage the poison spit works quite well.
The Swamp Spirit poison reduces enemy attack by about a third, lasts 3 turns, only costs 5 rage and has a Rest of 1 (when upgraded). The actual damage done by the spit is negledgable.
It also stacks with normal poison from snakes / alchemists.
________
VMX1200