Or you could make a tower that requires the reserch then add the bouncing tower to upgrade from that tower and do these steps to enable the bouncing tower to use a bounceing attack
Set the Area of Effect for Full Damage = area where bouncing attack can occur
Set AoE Targets = targets you want to be hit by the bouncing attack
Set Damage Loss Factor = The percent of damage lost with every bounce (.20 = 20% less damage on next bounce)
Set Maximum Number of Targets = maximum number of bounces
Make sure the weapon type is set to Missile (Bounce)
Here is a much more complicated way to do this with using abilities
A dummy spell based off of Channel. This is the ability you give to your unit/hero, and its only purpose is to start the trigger.
A custom version of whatever spell you want to be cast on the targets (You said that you want Banish). This custom spell should cost 0 mana.
A dummy unit that has your custom ability and no model file; giving it a model path of _ (an underscore) works well. You probably also want to give it no shadow image and the Invulnerable ability so that it can't be accidentally killed.
You then need this for a trigger:
Code:
Event A unit starts the effect of an ability
Condition Ability being cast equal to YourDummySpell
Actions Pick every unit within 400 of (target point of ability being cast) matching (owner of matching unit is an enemy of owner of casting unit) and do Create 1 DummyCaster for owner of casting unit at position of picked unit Add a 1 second expiration timer to last created unit Order last created unit to cast Banish on picked unit.
Hope this helps.