After the tragic events in the Slaughtery, you’ll be back in the inn. Your other party members will leave the party except Miki since there's still battle raging outside. There's no time for mourning; after talking to her, move Fidel to make him move out of his seat then head outside the inn. Your other party members will regroup with you at the same time. Defeat the enemies outside the inn then head to the town center. There, Der-sul and his men are waiting. You can finally settle the score against him once and for all.
Der-sul is powerful but if you’ve trained your party well at this point, you can pretty much soak up whatever damage he inflicts just fine. He has a strong, wide-area attack that can hit multiple targets up front and can easily break your guard. You can block it or evade to his blind side. He also has a powerful area-of-effect move called Spicule where he jumps in the air and the impact upon his landing will cause an explosive blast that will deal multiple hits and send your characters airborne. He will do this at least 2-3 times during the battle. When you see him jump, you have to try getting out of the blast area as soon as you can.
If Fiore has Sacred Pain learned, it’s a good way to lower Der-sul’s elemental resistance and allow Fiore and Miki to deal more damage with their elemental attacks. Der’sul is a slow but solid attacker. He doesn’t perform combos and will have a lot of openings but he can deal solid damage with his strikes. That said, just keep him away from Miki and she’ll keep everyone healed up especially with the best healing and Support Roles you have right now.
After the battle, you can’t go anywhere until you trigger another scene. During this scene, Fidel will receive Daril's sword, called the Insigvinon. (Sadly, this is not a unique sword since you can easily craft this via Synthesis) Head to the exit leading to the Resulian Plains to meet up with your other party members. With Relia back, it’s time to set her role and equipment again. Your next destination is Santeroule.
Before leaving, make sure to to trigger some Private Actions inside the town. Exhaust the private actions available in Eastern Trei'kur and you’ll be able to unlock Relia’s "Happy Together" PA. Viewing this PA will unlock the Equal Opportunist role. Once ready, leave the town.
Tip: There are new missable Private Actions in Central Resulia, Myiddok, and Sthal so if you don’t want to miss anything, might as well make the necessary detours to view these PAs. A private action between Emmerson and Fiore in Myiddok will unlock the Spendthrift role. In the other hand, there are also no new sidequests available from the bulletin boards.