Celestial Slayer Achievement
Defeated Gabriel Celeste.Gabriel is found at the end of the Cave of the Seven Stars bonus dungeon. After beating the game, you will save once again and be placed back before the final boss. However, there will be two new teleporters: the east will transport you to the Calnus, the west to the Undying Dragon. Defeating the Undying Dragon will give you the Darkness Ring which will unlock the Cave of the Seven Stars by breaking the light-seal door in the Colosseum's west wing.
For more information on this dungeon and boss, see here.