Go into the go into Hermit Bar & Inn to accept the quest, then head out into West Languedoc Canyon Road and you’ll stumble across Clark. After the Cutscene you’ll need to take down Great Abyss. To do this, activate Ebon Dragon to increase your Critcal Hit chance by 50% and then use any S/A Crafts to deal maxium damage. Once the enemy is in the break status you’ll want to make use of everyones S-Crafts to finish it off.
After you’ve defeated the boss the Quest will complete and you’ll be given a Celestial Balm EX and 3000 Mira for your reward.