The Moonwatchers, who name themselves Lunarchs in their own liturgy, are a secretive cult built on a single conviction: the Third Moon was real, it was taken from the sky against its will, and one day it will come back. They hold to what the astronomers call the Literalist View of the Killing Moon. The Killing Moon and the lost Third Moon are laid out in full under Stars, Suns, and Moons; the short version is that most scholars treat "Killing Moon" as a grim nickname for the destabilization periods, while the Lunarchs insist on a literal third body, damaged and drifting somewhere beyond the planar stack, that screams back through Alaria's sky every few thousand years.
Doctrine
For a Lunarch the destabilization periods are not catastrophe. They are homecoming. The moon is trying to return, and each cycle brings it a little nearer before the world shoves it away again.
The cult builds this hope on a fact no priest disputes. Prayer moves the heavens. When the northern monasteries sing loudest, Bryn carries the sun closer to the north, and the seasons turn on which congregations pray hardest. The Lunarchs reason that the same pull, aimed upward and outward and timed to the dark of both moons, can draw the Third Moon home, provided enough of the faithful call for it at once. Orthodox doctrine holds that no single cell can manage it. The moon returns only when the whole scattered cult cries out together, which is the closest thing the Moonwatchers have to a shared purpose.
What the return will bring is the cult's oldest argument. Most expect a cleansing: the weak and the corrupt scoured away, the faithful left to inherit a washed world. A smaller current expects the boundaries between planes to dissolve and mortals to ascend past them. And a stubborn minority believes the moon does not want to come back at all. It is hiding, they say, and the destabilization periods are not its approach but its death-throes, the long slow expiry of something wounded in the void. These last few do not pray the moon home. They want to find it and heal it, and they regard the rest of the cult as accessories to a murder still in progress.
Practices
Many Lunarchs keep one eye permanently covered, bandaged or patched or in the older cells ritually blinded. They call it the blindfold covenant. The covered eye is saved, held back unused so that it might be the eye that finally witnesses the moon's return. Some of the senior faithful put out both eyes and claim they see the coming moon more clearly for it.
Initiates take crescent scars, usually on the forearm, the shoulder blade, or the shaved scalp. The count and arrangement of the crescents reads as rank and sect to anyone who knows the grammar, which is more or less only other Moonwatchers.
The cult's largest rites fall on Hollownight, the double new moon when neither Auris nor Nyxara touches the sky. On that darkest night the faithful climb to high ground and chant the moon back toward an empty heaven. The choice of night is doctrinal: with the two living moons gone under, they believe the third has the clearest road home.
The sky is not empty tonight. It is occupied by something you have been taught not to see. Open the covered eye. Look up. Call it down. — opening line of the Hollownight call, as recorded by a Sennos magistrate's informant
They also collect moonstones, fragments they hold to have fallen from the original moon. Most are plainly ordinary rock, a few are meteorites, and a small handful are genuinely anomalous, matter no assayer can name. The prized ones are said to glow faintly on Hollownight, or to sit wrong in the hand, a cold unease that has nothing to do with temperature.
Organization
There is no head to cut off. The Moonwatchers have no central authority and no fixed seat. Cells form and dissolve on their own, and the only thread between them is the Heralds, traveling preachers who carry doctrine and moonstones from one group to the next, along with whatever news travels the road with them. Stamp out a cell and the others never feel it. This is why the cult has survived centuries of intermittent persecution while never growing large.
The cells sort loosely into four readings of what the moon's return means. The Cleansed await the purifying apocalypse. The Transcendent await planar dissolution and ascension. The Mourners hold the moon to be dying and in need of rescue. The Wrathful believe it was murdered and want vengeance on whoever did it, though they have never agreed on a suspect, and have on occasion turned that suspicion on each other.
The Dead Moon Tribe
The Dead Moon Tribe of Moonwood draws power from Nyxara and carries a lunar fixation that looks, from outside, like a cousin of the cult's own. The two are connected, but how is genuinely contested, and both sides refuse to speak of it. The tribe keeps its own account; the matter is set out there rather than repeated here.
How dangerous they are
Most governments file the Moonwatchers under nuisance. The beliefs read as delusion, the rites look harmless, the numbers are small.
The record is less reassuring. Astronomers who publicly denied the Third Moon have been found dead. Observatories have burned and celestial archives have vanished. There are credible accounts of rites that called for mass sacrifice to feed the returning moon, and of cells stealing exactly the kind of anomalous artifact that might, if the literalists are right, be a true fragment of it. The Lunarchs believe what they say without reservation, and a faith with no doubt in it and no center to negotiate with is a hard thing to predict.
Game mechanics
For the GM
Moonwatcher contacts work well as keepers of obscure celestial lore, as antagonists mid-ritual, or as unlikely allies when something threatens "their" moon. A cell holding a genuine moonstone is a small treasure-and-trouble hook on its own. The Mourner faction, who want to heal the moon rather than summon it, are the easiest Lunarchs for a party to work with and the easiest to misjudge.
A Moonwatcher player character is unusual but playable: someone who believes the world will end, eventually, and has complicated feelings about whether to want it.