On the west side of the labyrinth you will find a lone, sealed portal. As you progress through the Sagus Cliffs location, it will become more and more unstable. When you approach it and decide to open it, you will receive a new quest - A Wrested Mind. Head over to the new location.
When you arrive, a confrontation with the Sorrow Fragment awaits you - you will reach the location with Diviaticu, Kamose and Villon. You can decide, if you want one of the men to use a powerful spell, which will destroy the beast (he will then not engage in combat and will order you to protect him), or simply to take part in the combat. It is better to choose the second option - the Sorrow Fragment is not very dangerous, it does have 99 health points, but the combined attacks of all of your characters will reduce that to zero in no time.
After the fight you will gain 25 XP, the apparitions will vanish, and the quest will be a success.