Primus is the ultimate Modron hierarch, looking like a gigantic humanoid rising out of a pool of light. One of its hands is hidden in radiance (a portal to Arcadia), the other in darkness (a portal to Acheron). Its body is joined to the minds of every modron via something called the Infinity Web. It has all the powers of a greater god.
Primus was slain by the protodeity Tenebrous in the prelude to The Great Modron March. When Tenebrous disconnected from the Infinity Web and left Mechanus, one of the secundi who serve as viceroys of the four quarters of Regulus was promoted to become the new Primus.
Because Primus was the equivalent of a greater god, it did not truly die when slain by Tenebrous; instead, it became a Vestige caught between planar dimensions. It can now only be contacted by a Binder using Pact magic.