Primarch was the title for the leader of a faction of the Yod-Thot in the Breen Imperium during the 32nd century. There were six Primarchs in total.
Ruhn was the Primarch of the Sixth Flight. Another Primarch was Tahal who led the Third Flight. (DIS: "Mirrors", "Erigah", "Lagrange Point")
The six Primarchs were involved in the Breen Civil War, each seeking to claim the throne of the deceased Breen Emperor for themselves. (DIS: "Mirrors", "Erigah")
After Ruhn was killed by Moll, the other five Primarchs learned of it within hours and moved to consolidate their claims to the throne. Tahal set out to claim Ruhn's surviving forces for herself to bolter Tahal's claim to the throne. (DIS: "Lagrange Point")