When the Elements of Harmony banished the Darkness from Nightmare Moon, they believed themselves to have garnered an absolute victory.
But as with all things in the universe, there must be balance. No Light with out Darkness, no Wrong without Right.
The Darkness has been biding its time in the one place in Equestria where magical Ley Lines converge, where all the residual magic and energy is coalesced in to a single point.
The Everfree Forest.
Now stronger than ever, the Darkness has found a new host, and seeks to exact its vengeance, and rain terror on all of Equestria.