The Dagger of Venom is a magic item with the following special property :
You can use an action to cause thick, black poison to coat the blade. The poison remains for 1 minute or until an attack using this weapon hits a creature. That creature must succeed on a DC 15 Constitution saving throw or take 2d10 poison damage and become poisoned for 1 minute. The dagger can't be used this way again until the next dawn.
The Poisoner feat (from Tasha's Cauldron of Everything) has the following feature :
You can apply poison to a weapon or piece of ammunition as a bonus action, instead of an action.
Would someone with the Poisoner feat be able to activate the special property of a Dagger of Venom as a Bonus Action rather than as an Action ?
Best Answer
Technically, probably not, but once per day isn't going to be a problem.
It seems pretty clear that the poisoner feat is referring to poisons like those created with the poisoner's kit. And technically, when you use the dagger of venom, you are activating a magic item, which applies the poison to itself, not applying a poison to the weapon yourself.
But, the dagger of venom effect is a once per day effect, and while this does make the dagger better, once per day makes it so that it shouldn't be an issue balance wise. I'll call it "up to the DM".
This would fix a problem I had with the dagger.
I've used a dagger of venom while playing a rogue. It got some mileage during assassination attempts, but was mostly useless in combats where preemptive strikes didn't happen. To me, the cost of an action was far too great for the reward, and so the dagger of venom turned out to be a very specialized tool that I was only able to use a few times. It would have been nice to have a guaranteed once-per-day use by allowing it to function with a bonus action instead of an action.