Thurisaz
Thurisaz, also known as Thorn, is a former magus of Blackthorn. He was a founding member of the covenant of Blackrose.
Capture of Ozerk
Ozerk of Stranggore was captured, cursed, and imprisoned for many years in the ruins of Rosalba at Maiden Castle. After Agnes and Godwyn rescued Ozerk, some of his memory returned. He claimed Thurisaz, now known as Thorn, had been his captor.
It is now clear that Thurisaz/Thorn is the subject of the warning message meant for Ozerk, which Godwyn discovered in Release the Hounds:
the hunter is now the hunted beware the thorn of the black rose seek the light and return at dawn
Faerie Transformation
In the story of Ozerk's Revenge, Ozerk told Godwyn that the magus formerly known as Thurisaz was now a faerie called Thorn. He had undergone some kind of transformation. The name Thorn means "giant" in Old English, and Thurisaz is apparently a Latinized version of the same name.
Capture and Trial
Thorn was captured by Godwyn at the end of the story of Ozerk's Revenge. Godwyn brought him to the Stonehenge Tribunal of 1201 to face trial for robbing Ozerk of his Gift. This is complicated by the fact that, since he is now a faerie, it is not clear whether Thorn falls under the Tribunal's jurisdiction.