In 2016, on stage at Wembley Arena in front of thousands of adoring fans, Dr. Ruja Ignatova promised her followers a financial revolution. The future, she said, belonged to cryptocurrencies such as Bitcoin. And the Oxford-educated, self-styled cryptoqueen vowed that she had invented the Bitcoin Killer. OneCoin would not only earn its investors untold fortunes; it would change the world. By March 2017, more than $4 billion had been invested in OneCoin...