Nomus Rhy'tan was a Lord Chaplain of the Salamanders during the Great Crusade and Horus Heresy.
When Chaplains came to the Salamanders Legion, it fell upon fertile ground as the spiritual discipline of the Legion and the philosophy of the Great Crusade were already considered greatly important to the Salamanders. The direct promulgation of the doctrines of Vulkan, shaped by the culture of Nocturne, were already being distributed by the 'Voices of Fire' - a body of chosen Legionaries of which Nomus Rhy'tan was already among.
Many of these men went on to become the Legion's first great generation of Chaplains and Rhy'tan already was highly regarded as a confidant of Vulkan as well as a renowned orator. Rhy'tan was left behind by Vulkan before the Drop Site Massacre with the duty of overseeing the Legion's Neophytes and acting as Regent in his stead. Thus he survived the massacre of his Legion, and was the one who had to pick up its fragmented pieces after Vulkan went missing.
Nomus Rhy'tan proved a key loyalist commander when Death Guard forces later assaulted Nocturne.