(Passive) Hero's physical attacks apply Poison 1; as a single action, she can throw a bottle of poison that poisons all units in a small area.
Both ranged and melee physical attacks will apply the poison effect. Other physical damage sources like Jumpjaw traps or magic damage like Flashcone will not apply poison. Update 0.13+109 Darrel Trace reworked this ability to include an AOE poison bomb.
Quellingmoss is particularly effective against enemies with high armor, as poison bypasses all damage mitigation.