That is the -acc debuff? I slotted it with 2x acc-debuff and use it mostly on boss/lt's. I think they hit my bots a bit lesser.. but realy effect, not realy.
50)Sinergy X/(50)Mika.
(50)MaceX/(50)Encore
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Actually, Envenom is not that bad. It won´t necessarily save your weakest pets from that Lt. who is +3 to you, but it will sure make a difference if that Lt. attacks you or your human teammates.
If it has eyes, you can blind it, if it has blood, you can make it bleed, if it has a mouth, you can make it scream.
Envenom, thats the one that slows/stops the enemies End/health regen.
I havent used it much, as most fights dont last long enough for such a power to be useful, though I have used it on a few Capes.
Part of the problem is that all the enemy have vast amounts of endurance, so unless you have legions of endurance sucking allies it wony make any diff at all.
But then thats an issue I have with the game as a whole..Will we as player chats ever be as "nails" as any of our enemies? nope.
The best thing about this power is the defence debuff. I notice a difference after hitting an enemy with it. Its not an amazing power but it debuffs so many things so I think its worth it. Besides like I said the defence debuff part is worth it. 
I picked this power, as it sounded kinda cool....but i dont even notice the difference if i use it or not. to me it seems kinda lame, wish i went for weaken instead now...can anyone praise envenom? coz i cant