The Sunless Vigil is the long dark that followed an attack on the sun's core, in the deep ages after Bryn woke and before any mortal kept a calendar. For roughly five hundred years the greatest of the titan-crafted stars gave no warmth. The world did not freeze to nothing, but it came close.
The cause was a Xaphkiel. In the era when the shadow-born angels and their light-born rivals still waged their war among the stars, the Xaphkiel made a practice of smashing celestial-glass cores and seizing the dead stars for themselves. The sun was the largest core of all. One of them reached it. Where it had extinguished lesser stars cleanly, the sun fought back, because by then the sun was no longer a mechanism. Bryn was awake, and Bryn did not want to go out.
Koras alone
The struggle corrupted the core rather than killing it. The celestial glass clouded and cracked along a single seam, and the light guttered down to a dim ember that gave neither true day nor honest dark. The Starborn keep watch over the stars in their nightly thousands, but the sun has only ever had one tender, and Koras would not leave it. No other Starborn could survive the heat of the core even diminished, and none was permitted near it. For five centuries Koras worked the seam alone, drawing the corruption out of the glass a thread at a time and feeding the ember back toward fire.
The core at the depth of the Vigil: a sphere of celestial glass the size of a small moon, gone the color of bruised pewter, a single black seam running pole to pole and weeping cold light. A lone figure braced against it, hands pressed flat to the glass, the heat that should have killed any living thing pouring through and around without touching them. No sound but the slow tick of the glass re-knitting itself, one thread at a time.
When the sun caught again, it was not the same sun. The seam never fully closed. It holds, but it holds because it is tended, and Koras has not left the core unwatched since. This is the vigil the period is named for, and it did not end when the light returned.
What it left
The Sunless Vigil is why the inversion between Koras and Bryn is permanent. Before the corruption, Koras tended a construct. After it, Koras served a person who had refused to die, and the two of them had spent five hundred years keeping each other alive in the dark. The bond between them dates from the Vigil and no mortal has ever stood close enough to describe it.
The event also gives the Dawnless their oldest argument. The world survived five hundred years of a sleeping sun, they point out, and rebuilt afterward. If Bryn was allowed to rest once, the suffering was not unbearable, and the sun's eventual stillness need not be the catastrophe the theocracies preach. The sun-tax churches answer that the world nearly died and was saved only because one Starborn refused to stop fighting. Both sides are arguing about the same five hundred years.