Simun Brambling is a prisoner of Madman Lugos at Kaer Muire on Ard Skellig. He seems strangely comfortable with being in such a place and he can help Geralt of Rivia get out of there, in exchange for a favor to be returned at a later date.
If the witcher takes Simun's help to escape, he'll return outside the New Port Inn at Kaer Trolde asking for the debt to be repaid and to meet him at a hut on the lake near Blandare without specifying why. If Geralt agrees and heads to the hut, Simun will antagonize Volund before asking Geralt to kill him. Volund will then tell Geralt that Simun killed his brother by tricking and betraying him, leaving it up to the witcher to honor his debt or not.
If Geralt fulfills his debt to Simun: The witcher and Simun kill Volund along with one of his warriors. Simun reveals that Volund had hunted and sent assassins after him wherever he went on the Isles. Debt repaid they part ways.
If Geralt refuses to fulfill his debt: The witcher and Volund with one of his warriors kill Simun.