Domains: Secrets, hidden knowledge, the unspoken.
Era of ascension: Golden Age of Man (~145,000 years ago). Cohort: dead. Died: God War & Long Winter (~75,000 years ago), when the cold compressed every active initiation to nothing and his cult died with its last full initiate.
Worshipped by: The esoteric orders of the Golden Age, the mystery cults whose rites were known only to those who had passed the initiations to be told them, and the intelligence-services of the great polities — who used his shrines as cover for the practice of withheld knowledge they were professionally engaged in. No living culture maintains his worship. (The worships edge is authored downstream on the worshipper, not here.)
Therríneth was knowledge not shared. Graelith was the library — the deliberate effort to make knowledge durable across hands. Brethis was prophecy — knowledge that a daemon revealed, sometimes to its receiver's cost. Therríneth was the third structural position: knowledge that humans concealed from each other, deliberately, as a religious act. His mystery cults required years of progressive initiation to learn what the higher initiates had been told; the doctrinal premise was that a secret known by everyone is a secret destroyed, and that withholding was itself the worship.
His cult was small in numbers and disproportionate in cultural weight. Almost every major Golden-Age polity had at least one mystery order. The orders cross-trained their initiates with each other quietly — there are texts in which a senior initiate of one cult could attend the inner rites of a neighbor cult by sign, without ever having spoken a word of either order's secrets aloud. His priesthood ran the most secure intra-polity diplomatic channels of the Golden Age on this basis. Romath's institutional law could not match it.
The Winter killed him through his own bottleneck. Initiation required twenty to thirty years of cumulative formation; the cold killed initiates faster than masters could complete them; within two generations the orders held only their senior priests and a thinning tail of mid-formation students; within three, only the seniors; and the seniors died with the secrets in them, refusing to release the formation in extremity on the doctrinal grounds that broken initiations were no initiations at all. His name is in the records of the polities who used his orders; the secrets are not.