I have a story in mind.

Yveltal is in love with the beautiful Articuno, and he believes that she loves him back. However, when he discovers that she never felt the same and has found a mate, he is consumed by envy. He enters the training of a mysterious Honchkrow deemed "The Corruptor," where he learns many sneaky, evil tactics. One day, Yveltal goes too far, and Articuno enters a fit of rage. In order to make up for his deeds, Yveltal must get help from an old friend, sacrificing his life to return peace to his home.