The outworlders' club housed physically inside the awakened final boss — now a torture-trap they can't die out of.
Club Scolopendra is the club where the outworld 'tourists' (former crawlers, sponsors, and other guests) are housed on the sealed endgame floors — a space physically located inside the body of the awakened boss Scolopendra. With the Nothing broken, the system AI won't let anyone eject: guests can be hurt and 'killed' but are instantly revived back in their club rooms, where escaped gods, demons, and monsters from hundreds of former quests now stalk and endlessly torment them. Getting from the eighteenth floor to safety on the sixteenth requires crossing the lethal seventeenth, where anyone who tries is minced and revived.
Notable 'guests' include Otto of Otto Banking Unit.
A club interior nested within the guts of a colossal living centipede boss; a revolving nightmare of revived guests and loose monsters.
The inescapable club-prison inside the final boss where trapped outworlders are endlessly killed and revived.
Also appears in books you've already read — spoiler-safe to their endings: