Ethereal Executioner Achievement
Defeated the Ethereal Queen.The Ethereal Queen is found at the end of the Wandering Dungeon bonus dungeon. Once you have cleared the Cave of the Seven Stars, take the elevator in En II back to the Road to the Old Sanctuary and speak with Barago, Keeper of Past Knowledge. He will guide you to the Wandering Dungeon.
For more information on this dungeon and boss, see here.