Need Some help

Wonder how Damage Target trigger works

Unit-Cause(Trigger Unit) to damage (trigger unit), dealing 500 damage of attack type spells and damage type fire.

which function spot is player and enemy that work best for it.|||ah k thanks. Need help with this trigger so caster don't take the damage.

Events

Unit - A unit begins casting ability

Conditions

(ability being cast) Equal to Thousand Needles

Actions

Unit-Cause(trigger unit)to damage circular area after 0.00 seconds of radius 475.00 at(position of(casting unit),dealing (Random real number 0 to 1000) damage of attack type Pierce and damage type Enchanced

don't know which part i went wrong on the trigger