Well if your actions happen if a unit us by a normal attack, there's no way to detect the dammage. The only solution is by orb effects (which is actually how Vexorian's attack detect engine works). This may not be 100% correct, but it'll do:
1.Make a new custom buff based on Slowed. Don't bother changing anything except the name and remove Art-Target model.
2.Make a new custom ability based on Item Attack Frost Bonus. Change Data - Damage Bonus to 0, Data - Enabled Attack Index to 0, Stats - Buffs to the buff you just created. Remove the Art - Missile Art, Art - Target and Art - Special models.
IMPORTANT - Change Duration - Hero and Duration - Normal to 0.1
3.Give this ability to your unit.
4.The trigger :
Code:
GrowTree
Events
Unit - A unit Dies
Conditions
((Dying unit) has buff (The buff from step 1)) Equal to True
Actions
Destructible - Create a Ashenvale Tree Wall at (Position of (Dying unit)) facing (Random angle) with scale 1.00 and variation 0
Of course you can change the actions to whatever you want.
The things are diffrent if the damage is done by the trigger, because it won't leave a buff. In this case, things are a lot easier. Just put that right after the damage action :
Code:
If (All Conditions are True) then do (Then Actions) else do (Else Actions)
If - Conditions
((the unit that you just dammaged) is Dead) Equal to True
Then - Actions
Destructible - Create a Ashenvale Tree Wall at (Position of (Dying unit)) facing (Random angle) with scale 1.00 and variation 0
Else - Actions
