Changing Ability Targetting

How do you change the targetting of an ability to accept only locations on terrain? I want to make Carrion Swarm deal damage to practically anything, but only allow the user to target the ground and not click on a unit to have it automatically go in the unit's exact direction, wherever the unit goes.

You can create a spell based on channel with Point Target and then use some dummies to cast carrion swarm.

You can't change targetting of ability itself? Shame.