To start this sidequest, put Fides in your active party and fly to Gatherington. There is a man sitting down in the pub, which is the building where the blacksmith is standing, who will initiate the sidequest once you talk to him. He tells you about a cursed Spritnite, but doesn’t know where it exactly is located. Upon stepping outside, there will be a short scene, where you learn that the kid who stole it is inside of the Fridging Caves.
If you do not wish to use Fides, you can switch him out for another character to place in your party. Anyways, once you’re ready, go to the Fridging Caves and make your way to the save point, right before where you fought the Aurorean Tiger in the main story. You’ll encounter a familiar face in the former boss’ arena and once the scene is over, you’ll be face-to-face with a boss!
BOSS - Dysphormic Monster
The rematch against this dragonic foe is pretty much the same as it was during the main story. You’ll likely want to boost your HP with a Purikka Cobbler Salad before the fight, since it will help in surviving its attacks a little better. The majority of its attacks hit more than one person, so you’ll need to keep an eye on everyone’s health at all times and heal accordingly.
Dragonic Tail is probably the main attack you’ll want to watch out for throughout most of the battle, since it is guaranteed to hit everyone. Setsuna is amazing in this battle because of both Inspire and Cure II; make sure you use the former with Momentum, since it will make the buff last longer. The only major difference with this rematch is once the Monster’s health gets below a certain threshold, it can use Dragnarok at any time, instead of just upon death.
Your reward for being victorious will be the Dragnarok Spritnite for Fides, as well as another scene.