Domains: The moons (the twinned cycle of Auris and Nyxara), tidal magic, lunar timing.
Era of ascension: Golden Age of Man (~175,000 years ago). Cohort: dead. Died: God War & Long Winter (~75,000 years ago), when the Winter's perpetual cloud cover hid the moons for years at a stretch and her cult, anchored entirely to the visibility of the 11- and 23-day cycles, lost the anchor.
Worshipped by: The moon-priesthoods of the Golden Age — seers, tide-readers, the small specialist orders of necromancers whose work was timed to the brief windows when the cycles of Auris and Nyxara aligned. No living culture maintains her worship. (The worships edge is authored downstream on the worshipper, not here.)
Olluréth was a single daemon paired to both moons, which is the doctrinally important detail. Auris and Nyxara are not gods themselves — they are resonances of the deep cosmological pair, Aurus and Nydus, and the canonical sources are explicit that the moons do not have their own divinity. What they had instead was Olluréth: a daemon of the worship of the moons, whose domain was the human practice of timing work to the lunar cycles rather than the moons themselves. The distinction is the only way the theology was coherent at all, and her priesthoods defended it carefully.
Her cult was specialist. The 11-day Auris cycle and the 23-day Nyxara cycle aligned in known but rare configurations, and her priesthoods maintained the tables that predicted them — the night of the doubled full moon, the dark when both crossed the horizon together, the rare seasons when the cycles ran in or out of phase. Necromancers used the alignment-tables to time work that wanted both moons at once. The seers of the polar polities used them for the long predictions that could only be made on the rare clear nights when both could be sighted simultaneously. Her shrines were observatories first and altars second.
The Long Winter killed her with cloud cover. The volcanic winter following the Kajiit eruption put thick particulate cloud across most of the world for years; in the high latitudes, decades. The moons were not visible. The cycles were not observable. Her priesthoods kept the tables for a generation in the hope of resuming, and then began to die without ever having sighted the cycles they had been keeping the tables for. The last predictive astronomical record of her cult is dated by event-correlation rather than by lunar reckoning, which her priesthoods had treated as a doctrinal impossibility — and which the keeper of that record acknowledges in a marginal note as the last act of the temple.
Ornath is the sky as atmospheric weather; Sorveth is the sun and the doctrine of revealed truth. Olluréth was the worship of what was up there at night, when the night was something more than dark.