A passive skill doesn't work with the event "Starts the effect of an ability". If you wish an active one, so that the trigger actually works, you can use Chain Lightning with targets set to 0 and damage set to 0 as well. Otherwise, you will need a Damage Detection System to check when the attacked unit receives damage from a source with Envenomed Weapons learned or you can add the hero in a unit group once the ability is learned (Unit - A unit learns a skill event) and check if the damage source is in that unit group (boolean comparison).