Discord, the spirit of Chaos was given redemption by Princess Celestia when discovering the magic of friendship. However, his true nature gets the better of him, and once again he uses his power for his own will. The elements of Harmony are protected from him, but there was nothing saying that the element bearers couldn't be eliminated.
Realizing that Discord has no intention of stopping and has rendered the elements useless, Celestia and Luna must face Discord in a final showdown. His powers are too great for them to fight when they are in their current form, but perhaps two alicorns are not needed. Maybe only one.
Inspired by the artwork by dreatos