maybe it would help you to modify the skill "pheonix fire"? It shots automaticly at targets in range of your hero. Give it an arrow missile and do a trigger like: If ability pheonix fire is casted on unit do: do 5 damage to target of ability, wait 2 second, do damage, wait, do damage, and so on, to simulate the poison effect. Dont know if this trigger works but this is the way i would try it.