It would really be better for you to create the trigger yourself. That way you learn more than if we just give you one. You should ask questions about how to create triggers, not ask for the triggers themselves, since this way is the best to learn triggering yourself.
Anyways, you could have an event that runs each x seconds and then adds all players to one force and picks a random one, then removes him from the force, teleports his hero to the arena and then does the same thing once again.
Then have a trigger that runs when a unit dies in the Arena Region to revive and teleport the contestants back to their base.