Could you give us a specific scenario? Did the enemy champion heal in the meantime? Who did you use it against? 

It's in general. I just see the poisen execute on the turn and I would expect to see about 25% of the champion's hp drop. I never see that happen, doesn't matter if its a boss or a crowd. I mean this would be crazy, but if five poisens are 25% of max HP, wouldn't clanboss drop like a tree? 

The Clanboss has a passive that reduces damage from masteries (Warmaster/Giantslayer), enemy max hp attacks (like Coldheart), poison and hp-burn.

Normal mobs should indeed lose 25% of their hp from 5 poisons. Could you underline your statement that isn't the case with some screenshots?

Hm.... I just tested this on some normal waves and campaign boses and it worked as expected with the 25% drop. I then tested Arcane Keep stage 15, and in the first wave, looks like all hit with 25% except Executioner. I didn't even see him drop, then heal. So I guess I have learned:

1. Yes its supposed to drop by 25%

2. Some champions still stop this from happening by healing just afterwards and other reasons id like to understand. (I don't see anything in Executioner's kit that would stop him from getting the 25% drop).

3. Some boses can have reduced damage from max hp damage attacks.

Im pretty satisfied for now, knowing this, but item 2 above would be helpful to know more about. I can try to get some screen shots, but it might be a while...logistics are a bit much for me at the moment. Thanks for the help though!

I made a test run right now. For me, that looks as if the Executioner in the Arcane Keep lost 10% hp from 2 poison stacks, as he should. 

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The observations made in game, in the heat of the battle, are no valid source. You may overlook something. There is an Arcanist in the waves of this keep. Maybe she put a continous heal buff on Executioner in your fight, so he lost 25% from 5 poisons, but healed 15% hp back immediately.

So what I have learned is the HP bar isn't divided evenly (red  arrow), but the time bar is (green arrow). So strange, wonder what those vertical lines on the hp meter are. But I think this just solves my post though: Yes the poisen reduce the hp by the percentages named. Thanks Skadi and Balltazer.

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The vertical lines in the hp-bar indicate the absolute ammount of hp. The more lines, the more hp. The hp-bar has the same lenght for all (non boss) characters in the game, so is the way to show how much hp they really have.