Mysterion appears in Telepath RPG: Servants of God.
While looking for foreign support for the People's Resistance of Ravinale, Duvalier meets Mysterion in Somnus. Mysterion tell Duvalier that Senator Festus will support the "revolution aboveground" if the Resistance rescues Festus.
"Thank you for all your help. The senator tells me that he is most pleased with you."