"Defeat Cornelius, Xin, Raiden, Enya, Beatrice, Caius in the Sanctuary. Whatever political convictions they may have, plotting to attack a city full of civilians is not going to be tolerated. Wipe them out to end this."
How to Complete
There are 6 opponents you have to defeat to complete the quest, each occupying one of the first six floors of Lardon(Sanctuary). Taking the quest also seems to respawn Iago with his 3 Bone Dragons on floor 13, but it's not necessary to defeat him to progress the quest and there's no additional reward for defeating him again (the game says you unlock the Bone Dragon recipe again, but if you already have one it's just a fake message). Defeating each of them unlocks a different Dragoon or Hydra recipe, the recipe received corresponds to the main monster in that opponents army (see Monster Recipes for details)
Opponent details: all quest monsters seem to have status immunity for this quest
(bonus, not needed for quest) floor 13: Iago - 3xBone Dragon
Turning in the quest gives a Golden Egg