Oathbreakers
The Ward's one unforgivable crime. (Creative interpretation.)
Canon status: creative interpretation.
The crime
An oathbreaker is a warden who has broken the Warden's oath — abandoned a post, doused a lantern deliberately, opened the gate against orders, or fled the wall. The Ward has exactly one punishment, and it is not death: it is unmaking. The oathbreaker's name is struck from the rolls, their lantern broken on the wall steps, and no warden speaks to them again. In a city that belongs to the Ward at night, that is a colder sentence than any cell.
Why it matters to Nyra
The Warden's line on the subject — "Oathbreakers are not forgiven. They are answered." (character voice; creative interpretation) — is the hardest thing she says, and the Ward repeats it precisely because it costs her to say it. Mercy is a daytime virtue. The night has different arithmetic.
The empty chair
At Gatewatch there is a chair no one sits in, facing the wall. It belonged to the last oathbreaker, forty years gone. The Ward keeps it empty the way other cities keep monuments.