The Ruby Star unlocks Art Attack.
Art Attack is a Special Move Mario gets after finding the Ruby Star in the Creepy Steeple. It recquires four dots of Star Power to execute. With it, you can make the Ruby Star fly around the enemy with the Control Stick, leaving behind a red line that encases the enemy and then breaks, causing damage. You can do a maxium of 15 damage if the enemy is totally covered in the circle.