Tower of Mastery is a tower located in Shalour City. It is currently run by Gurkinn and the successor is Korrina. It is where the player can receive one of Korrina's Lucario and the Mega Ring at the top of the tower after the player defeats her at the Shalour City Gym and then, tested with Lucario against Korrina's other Lucario.
The origins of the Mega Evolution first began when a trainer came to Kalos, accompanied by a Lucario, and found 2 strange stones. One of them reacted to the trainer's Lucario resulting in the first Mega Evolution in history.
The Shalour City Gym is located inside the Tower of Mastery in the anime. Ash Ketchum challenged Korrina for a gym battle which Ash eventually won after a tough battle against Korrina's Lucario, earning his third badge.