Flask of Venom

A coating for melee weapons, and whips count for this, that leaves acid venom on whatever you hit. The debuff ticks for real damage and stops the target healing, which is what makes it worth carrying against enemies that regenerate faster than you can chip them down. Melee weapons that throw a projectile pass the effect along, so swords with beams and flails apply it at range as well.

One flask at a time. Drinking another wipes the first, so pick before the fight rather than topping up mid arena. The buff carries through death, which is the reason to drink before the first attempt at a boss instead of after it goes wrong.

Because the venom does its own damage on a timer regardless of the weapon, it adds proportionally more to a fast weapon with weak individual hits than to one enormous slow swing. Against high defense targets it is also the part of your output the defense does not reduce.