Themion served as a lieutenant within the Eriaduan judicial forces during the Yuuzhan Vong War, specifically in 26 ABY. Described as a tall, gaunt figure with a weathered face and a blackened eye, Themion also held strong Humanocentricist beliefs, as demonstrated by his derogatory description of Rodians as "a vile and murderous lot".
Secretly, Themion collaborated with the Peace Brigade, a government allied with the Yuuzhan Vong. In one instance, he orchestrated the arrests of Jedi Anakin Solo and Tahiri Veila for allegedly participating in a brawl where Brigaders attacked the Rodian Jedi Kelbis Nu. His true motivation was that he knew the Yuuzhan Vong would target Yag'Dhul next, due to its strategic location at the intersection of the Corellian Trade Spine and the Rimma Trade Route.
During their imprisonment, Themion ensured Anakin and Tahiri were placed in separate holding cells before personally interrogating them. During Anakin's interrogation, Themion resorted to physical violence when the young Jedi refused to cooperate. This act enraged Tahiri, who sensed the assault through the Force. Driven by her anger, she tapped into her Force abilities to break free from her cell and subsequently stormed into Anakin's cell.
Brandishing a blaster, Tahiri unleashed the Warrior caste battle cry, Do-ro'ik vong pratte, before forcefully slamming Themion against the cell wall. She then restrained him using his own stun cuffs. As another guard entered the cell, Anakin swiftly incapacitated him by throwing him against the corridor wall, providing the two Jedi with the opportunity to escape and retrieve their lightsabers.
Their escape involved fighting through more judicial officers stationed outside the prison and commandeering a police speeder. Ultimately, the two Jedi reached the spaceport and successfully fled off-world alongside their fellow Jedi, Corran Horn.