What It Does
The line, held in the light — the light is the answer, and the holding is the whole argument, and the one who holds is the one who is still standing. It is the master's light spell: the line holds by the light, and the light is the one who answers, and the answering is the one the dark has to stand in.
The Cost
Circle 4 · Light · Exchange: reagent (silver, named) — casting time two rounds. On the ladder in The Veil, the reagent rung is the price.
The Failure
The silver is named wrong — the reagent is the forge's, not the one the spell wants — and the line is held, and the line is the one who is still standing, and the one who is standing is the one who was already the gap.
In Stories
The master's light spell; the one the camp casts when the line is the argument and the holding is the one who has to stand.
The door's scale anchor is Luminary — the dragon casts it without thinking, which is how a reader learns the shape of the thing.