Copy either storm bolt (target) or cluster rockets (area of effect) and modify the ability fields:
- Change to unit ability (if you don't want a hero ability)
- Change the damage to 0
- Change the duration to 0.02
- Change the range to 999999 (shift+click should allow you to enter large values)
- If you chose cluster rockets as base ability, change the number of shards (missiles, whatever it's named) into 1