Event - A unit is attacked (or GDD_Event becomes equal to 0.00 if you're using Weep's GUI Friendly Damage Detection System)
Condition - Attacking unit has item of type *YourItem*
Actions - set your position, Create a Dummy unit on that position, add 'War stomp' to Dummy Unit, Add short expiration timer to unit, order Unit to cast 'War Stomp', remove the point.
Anyway, that would be really weird. I'm guessing you want to create the special effect, or deal Area Damage... Both have better ways to do than casting WarStomp.