Fire Emblem Valentia

From the introduction:

Many years ago, the Divine Dragons on the continent of Archanea ruled amongst the less powerful races. But one divine dragon, Duma, cause much chaos with his aggressive tendencies. Many times he was told over and over again to slow his warlike activities. Every time, Duma ignored these orders. The dragon ruler Naga exiled him, for she knew the fate that eventually befell all dragons who walked the path of war. To enter in a mindless state of near-permanent rage and impulse. She permitted Duma’s sister, Mila, to follow him to the continent. She also handed Duma a blade carved from one of her own fangs, so that when this fate befell Duma, the humans would have a means of slaying him.