Dragons are known for being big strong creatures, to the pointed where they're getting pretty violent. Dragon Lord Ember, however, has been wanting to make a piece connection with the ponies and other kingdoms of Equestria.
Yet there's one dragon who's not wanting to team up with the others but rather take over all Equestria starting with the Dragon Lord. To do this, he uses the power of a stone to erase Ember's memories. Now that she's no longer knowing she's in charge the dragon raises to power and no dragon can stop him. Keyword, "Dragon."