Explosive filler packed into Carl's barrel bombs alongside gunpowder, capable of leveling an entire city block.
Carl's barrel bombs were crammed with gunpowder and Hobgoblin Dynamite plus delay-detonated Smoke Curtains, packed in a way designed to cause the most destruction possible. He described them as unstable as shit — the mere presence of a leaning drunk caused one barrel's integrity to slide downward — but a single barrel could take out an entire block.
Four of them detonated on Zockau's pub row. The block burned, bodies littered the street, thick fog filled the area, every window in town blew out, the first rows of buildings down the residential street half-caved in, and the Desperado Club's entire façade collapsed. Carl and Princess Donut survived only because they were inside The Scuttlebutt, a real saferoom — and even there they were thrown across the barroom.
The explosive that lets Carl level a city block in a single stroke.
Also appears in books you've already read — spoiler-safe to their endings: