The Voth used their art on commanders in your army to bend their will, learn of your plans, and make them betray you, Thorn continued.
Thorn had been a part of the Voth's actions that caused betrayal. 
mnli hypothesis: Thorn had been a part of the Voth's actions that caused betrayal.  premise: The Voth used their art on commanders in your army to bend their will, learn of your plans, and make them betray you, Thorn continued.