That could work. I hate black and white border lines though (no offence to your idea though). Just because you have 1 extra experience point (on the border of level 19 and 20), why should the dagger suddenly stop working?
How about if the dagger does the creatures maximum health in damage, but it resisted by Poison Resistance. Mind you, that would make more sense if Resistances were percentages. And how frustrating would it be if something had a Poison Resistance of 5 so that they just survived the poisoned blow.
Idea three: It does 5000 Poison Damage (Poison Resistances apply) with it's poisonous strike (still has charge limitations as above).
