Ruhn was a Breen who lived during the late 32nd century. He was a member of the Yod-Thot, Breen royalty, and ruled as the Primarch of the Sixth Flight, one of the factions campaigning for the throne of the Breen Imperium. He was the uncle of L'ak, whom he raised from childhood. As L'ak was a descendant of the last Breen Emperor, Ruhn plotted to use his nephew to rise to the throne himself.
When Ruhn discovered L'ak in a romantic relationship with the Human Moll, he denounced his nephew for consorting with a "lesser being" and for assuming an "inflexible and weak" solid form with her. He demanded that L'ak kill Moll; instead, L'ak killed two guards and turned his weapon on Ruhn, prompting Ruhn to declare an erigah on him. Unable to bring himself to kill his uncle, L'ak wounded Ruhn so that he and Moll could escape. (DIS: "Mirrors", "Erigah")
After getting the map and apparently destroying Discovery, Ruhn prepared to break a tergun, a sacred Breen oath that he had made to Captain Michael Burnham not to destroy the Eternal Gallery and Archive which would draw the ire of both the Federation and the Breen down upon him. Moll goaded Ruhn into revealing that he only cared about power and cared nothing for his Scion, his oaths, or even his soldiers, causing Ruhn's forces to turn against him. Taking advantage of this, Moll disarmed Ruhn and killed the Primarch with his own weapon, taking control of Ruhn's ship and his forces for the goal of resurrecting L'ak and placing him on the throne of the Breen Imperium. (DIS: "Labyrinths")
Within hours, news of Ruhn's death spread amongst the other Primarchs who began using it to strengthen their own bids for the throne. In particular, Tahal pursued Ruhn's ship with her entire fleet, intending to take Ruhn's forces for herself, but was dissuaded by Saru. (DIS: "Lagrange Point", "Life, Itself")
In an alternate timeline, Ruhn used the Progenitors' technology that he got from Moll and L'ak to destroy the Federation. When a time cycling Michael Burnham and Rayner arrived on the derelict USS Discovery of 3218, Ruhn's dreadnaught was parked near the wreckage of the USS Federation. (DIS: "Face the Strange", "Erigah")
Ruhn was played by Tony Nappo.