Loom of the Black Mist

The Fortress of the Black Mist was never the source. Under it, down a stairwell the fortress's own builders bricked over, there is a loom. It is enormous, older than the fortress, and it is strung with breath: every person who has ever suffocated in the mist exhaled one last time, and that exhalation was caught, carded, spun into thread and woven. The mist rolling over the island is not weather. It is fabric, and the fortress was built to keep it from unrolling all at once.

The weavers are the dead who worked the loom in life and did not notice they had stopped being alive. They are courteous. They will pause, and apologise, and then resume. That is the register of the whole dungeon and the reason it unsettles more than the fortress above it: nothing here is angry with you, and nothing here is going to stop. It is a textile mill that never got to close, and the horror is the routine.