To approach the subject of red mercury is to journey into a comic-book universe, a zone where the stubborn facts of science give way to unverifiable claims, fantasy and outright magic, and where villains pursuing the dark promise of a mysterious weapon could be rushing headlong to the end of the world. This is all the more remarkable given the broad agreement among nonproliferation specialists that red mercury, at least as a chemical compound with explosive pop, does not exist. ... Legends of red mercury’s powers began circulating by late in the Cold War. But their breakout period came after the Soviet Union’s demise, when disarray and penury settled over the Kremlin’s arms programs. As declining security fueled worries of illicit trafficking, red mercury embedded itself in the lexicon of the freewheeling black-market arms bazaar. Aided by credulous news reports, it became an arms trafficker’s marvelous elixir, a substance that could do almost anything a shady client might need: guide missiles, shield objects from radar, equip a rogue underdog state or terrorist group with weapons rivaling those of a superpower. It was priced accordingly, at hundreds of thousands of dollars a kilogram. With time, the asking price would soar. ... Red mercury was a lure, the central prop of a confidence game designed to fleece ignorant buyers. ... When the Crocodile placed his order, Abu Omar said, the smuggler asked how much the Islamic State was willing to pay. The answer was vague. The Islamic State would pay, he said, ‘‘whatever was asked.’’ This was not the practical guidance a businessman needs. So the Crocodile sharpened the answer. Up to $4 million — and a $100,000 bonus — for each unit of red mercury matching that shown in a set of photographs he sent to Abu Omar over WhatsApp, the mobile-messaging service. ... the hoax has roots in an intelligence-service put-on, a disinformation campaign of phony news articles planted decades ago in Russian newspapers by the K.G.B. and one of its successors, the F.S.B.