The Primus is the highest religious function of the Triune cult. The Primus rules the Triune from the main temple in Kehjan.
In truth, the Primus was but a disguise of the demon Lucion, who secretly tried to establish a foothold for the Burning Hells on Sanctuary. After his 'death', the form of the Primus was used by both Lilith and the arachnid demon Astrogha.