EZ, heh, its in my 'most commonly used triggs tut.
Choosing a hero from (Circle of Powers)
*Event
Unit- A unit enters **region**
*Conditions (none)
*Actions
Unit-Remove (triggering unit) from the game
Unit-create 1 **hero** for (owner of (triggering unit)) at (center of **region**) facing Default building facing degrees
Trigger-turn of (this trigger)