Primus

From Diablo Wiki

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.