Discord, the god of chaos.
He like anyone else, has a dark side. Only his is one that should never be introduced to the world as its thirst for destruction is boundless.
When Discord was sealed away the second time his emotions hadn't had time to settle entirely, causing some leftover anger and hatred to carry over from the previous sealing.
Discord himself hadn't carried these emotions, rather they were passed on to his dark side to hold on to, accumulating over time.
Now as Discord waits patiently in his statue, his dark side has become increasingly aggressive, counting the days until it can break free and wreak havoc without the hindrance of Discord weighing it down.
Princess Celestia senses the disturbance coming from Discord's statue and sends a letter to Twilight and her friends.