Domains: Ice-glass, mathematical precision, the cold that preserves.
Era of ascension: Great Expansion. Cohort: current. Ystaeria emerged as the Istori refined the ice-glass forge into a process that could not function above a precise threshold of cold, and the craft and the cold became, in practice, indistinguishable from the patron of either.
Worshipped by: The Istori, in the forge-temples whose cold-vaults are the ritual chambers. The Traditionalists and the Industrialists are the same congregation; they pray to the same patron and dispute what the patron wants. (The worships edge is authored downstream on the worshipper, not here.)
She is the mathematical cold. The ice-glass forge requires a chamber held to within a fraction of a degree across measured intervals, and the work itself is a sequence of calibrated cooling steps that read more like an equation than a metallurgy. The patron is the equation. A forge-priest is also a mathematician; the temple has a library of derivations that double as liturgy, and an apprentice who cannot do the arithmetic cannot do the rite. The cold preserves what passes through it — the ice-glass that the forge produces holds its form and its clarity across centuries, longer than any other crystalline material in elven craft — and the cult's central claim is that preservation is what cold is for.
She is not P-2's patron Kryviel. Kryviel is winter as predation, the cold that hollows; she is winter as fixative, the cold that keeps. The two patrons share a domain on paper and an irreconcilable disagreement on what cold does in practice. An Istori forge-priest who encountered an Ikriel pixie would understand the worship-form well enough to recognize it as religion, and would also consider the religion theologically wrong in a way that requires no debate, only distance. The two cults do not contact each other and the patrons appear to prefer it that way.
The Traditionalist/Industrialist schism is the cult's live internal dispute. The Traditionalists hold that the ice-glass forge is a sacred craft and the output is liturgical — to be kept, displayed, used in further rites, not sold to outsiders who would not know how to keep it cold enough to preserve. The Industrialists hold that the patron's domain is preservation full stop, and the forge fulfills the patron's purpose when its output is in the hands of as many preservers as the export market can supply. Both quote the same derivations. Neither side accuses the other of irreligion, which is the schism's most distinctive feature; both sides accuse the other of arithmetic errors. The dispute is conducted in writing, in the same forge-temple library, on shelves separated by a single low partition. The patron has not intervened. The Istori say this is itself an answer.
The schism does not line up with the civil war, which is the thing both kingdoms find most inconvenient about it. Klevnaf is Traditionalist in its politics, and yet the forge-priest Elowir holds a Klevnaf congregation that argues the sacred Whitewood timber should be cut and the proceeds spent on the war — an Industrialist position pressed from inside Prince Taoinor's sacred-forest realm, by worshippers too devout to dismiss as mere profiteers. Istora is the reformist kingdom, the one that exports, and its Tuur forge-priest Aelrein is a Traditionalist who calls the sale of the Winterwood timber the scattering of the ancestors and will not be quiet about it. A Klevnaf Industrialist and an Istora Traditionalist agree with each other across the front line, and with their own rulers on almost nothing. The low partition in the library runs through both kingdoms, and it does not run where the border does.