After losing Queen Starfrost to the claws of Draxus, the King of Dragons, during the war against the dragons, Queen Moondust turned into a merciless killing machine to avenge her lover's death, slaying Draxus' entire army and the King of Dragons by herself, ending the war for once and all. This left a scar neither Equestria or the dragons could ever recover from.
Nopony ever heard of the Queen again, with her sister taking her place in the throne.