You can find all 5 Gorguath Lorestones during this mission as they are all located in the Gorguath dungeon. You need a Detect Hidden skill of at least 5 to find the hidden door to get the 5th Lorestone.
The first person who can kill the Grace Thresh that defeated the knight Sir Sagrell will claim his place in the House of Ballads. The beast rests in the Fae hollow of Gorguath to the south.
I defeated the Grave Thresh, legendary monster and enemy of the House of Ballads, to claim the seat of Sig Sagrell, the great Fae hero.