An un-defusable, city-leveling bomb permanently stuck in Carl's inventory — infamous across the galaxy.
Carl's Doomsday Scenario is the title-item nuke lodged in Carl's inventory from the previous floor — a city-leveling bomb that cannot be stabilized or removed and is always ticking toward detonation. Its existence is public knowledge; Brandon An's farewell notes that "everyone knows about that bomb you now got stuffed down your pants." On The Iron Tangle, once Carl gains the Determine Value skill, he finds it ranked "Pretty Damn Rare" and, tellingly, the least valuable item he owns — a constant, quiet menace he carries everywhere.
An unstable explosive device carried invisibly in Carl's inventory; never described in physical detail, only felt as an ever-present danger.
A permanent hazard and infamous signature item Carl cannot get rid of.
Also appears in books you've already read — spoiler-safe to their endings: