What you need:
-Create an ability based on war stomp, set its damage to 0 and area of effect to 0, rename it to ur ability and set the text descriptions and watever else u wanna change.
-Then create another ability based on carrion swarm and make it a unit ability. Set its damage to the damage of the ability you are making. Set MP used to 0 and distance to how far you want the aiming to be. Lastly, change the missile to the projectile that your unit is gonna aim and fire.
-Create a unit base on footman, give him the Invulnerable, the edited Carrion Swarm and the locust ability. Set under Art, its shadow image - Center Y, shadow image - Center X, shadow image - Height and shadow image - Width to all 0. Under combat, attacks enabled, set it to none. Set its movement type to fly.
Now for the triggering:
Events:
-Unit- Unit starts the effect of an ability.
Conditions:
-Ability Comparison- (Ability being cast) equal to (the edited war stomp)
Actions:
-Animation- Play (Casting unit)'s attack animation
-Unit- Create 1 (edited footman) for (Casting unit) at (position of casting unit) facing default building facing.
-Unit- Add a 2 seconds Generic expiration timer to (Last Created Unit)
-Unit- Order (Last Created Unit) to Undead Dreadlord- Carrion Swarm (Position of Casting Unit) offset by 100 towards (Facing of Casting Unit) degrees.
That should be all. Im gonna try it to be sure now.
EDIT: Yeah, okay its correct.