Poison Damage acts very slowly. Poison builds up in the target like Fatigue, but decays into HP Damage at a rate of 10% each combat round; nothing prevents this damage/fatigue. The type is chosen upon initial application, and some cause paralysis or stunning.

Stun Poison, instead of delayed damage, increases the target's Fatigue and is applied all at once. ( does size affect it now? A bug made this not the case in 5.)

Poison Resistance only reduces the initial application. Likewise, Poison Vulnerability only adds to the initial application.

A unit can possess only a maximum of 100 Poison at a time, just as they can only possess 200 Fatigue. Unlike with Fatigue, however, the excess Poison is completely wasted.

Poison cannot be run away from, nor can it be interrupted by completing the battle early. A unit who leaves the battlefield while poisoned has all of it decay off-screen. Any Poison that lasts past the 25 combat rounds of auto-resolving at the end of a battle also decays off-screen. This is why Assassins often carry a poisoned weapon.