Place Kir in your active party and head to Hiddbury, entering the tent in the central section of the town to trigger a scene with Hydor, the village’s chief. He mentions some powerful allies have come to help the village repel the monsters, so head to the Spritnite Pillar to meet up with them. Return to the village after the scene is over, heading back to the chief’s tent. With that done, get prepared (you don’t have to use Kir) and head back to the Spritnite Pillar to get into a boss fight.
BOSS - Syg, Quotra and Tronne
It is three-on-three for this battle and it might be challenging to you, depending on who you use. Although they might not appear to do a lot of damage individually, the damage can add up with all three sisters. Despite that, you can use food to increase your magic resistance (or HP if you please), as well as equip the Soul Prayer Support Spritnite (boosts magical defense) to help mitigate the damage you take from the sisters. You have three opponents to worry about, but one will quickly stand out, which is Syg (the one with the sword).
The reason that Syg can be a lot of trouble is that she has access to both Cure II and Life II, so not only can she heal (her heal recovers does 999 HP), but she can bring her sisters back to life with full health. Outside of those, she has Dark Mist and a physical attack called Monster Sword. When all three sisters are alive, they have a triple combo called Delta Attack, which surprisingly doesn’t seem to inflict too much damage (somewhere around 60-80 or so damage). Take down Syg first and you take control of the battle.
As for the other two sisters, Quotra (axe) is fire-elemental, so she has Fire (and Fire II if she gets low enough on HP), as well as a double combo with Tronne called Hellfire Hammer. Likewise, Tronne (hammer) seems to be ice-elemental, so she has access to Ice (and Ice II whenever her health goes low enough), in addition to the combo move called Icy Sentence with Quotra. None of the sisters’ moves have any special properties, so the fight is mostly about keeping your party in good health and taking out Syg first, then concentrating on the other two until they are defeated.
For winning the battle, you will get two things, a new sword for Endir (Darkmoon), and Kir’s ultimate Spritnite, Doom. You have finished another sidequest!