Until recently, Bill Hwang sat atop one of the biggest — and perhaps least known — fortunes on Wall Street. Then his luck ran out.

Mr. Hwang, a 57-year-old veteran investor, managed $10 billion through his private investment firm, Archegos Capital Management. He borrowed billions of dollars from Wall Street banks to build enormous positions in a few American and Chinese stocks. By mid-March, Mr. Hwang was the financial force behind $20 billion in shares of ViacomCBS, effectively making him the media company’s single largest institutional shareholder. But few knew about his total exposure, since the shares were mostly held through complex financial instruments, called derivatives, created by the banks.

That changed in late March, after shares of ViacomCBS fell precipitously and the lenders demanded their money. When Archegos couldn’t pay, they seized its assets and sold them off, leading to one of the biggest implosions of an investment firm since the 2008 financial crisis.

Almost overnight, Mr. Hwang’s personal wealth shriveled. It’s a tale as old as Wall Street itself, where the right combination of ambition, savvy and timing can generate fantastic profits — only to crumble in an instant when conditions change.

in a 2019 speech. “I couldn’t go to school that much, to be honest.”

Grace and Mercy Foundation, a New York-based nonprofit that sponsors Bible readings and religious book clubs, growing it to $500 million in assets from $70 million in under a decade. The foundation has donated tens of millions of dollars to Christian organizations.

“He’s giving ridiculous amounts,” said John Bai, a co-founder and managing partner of the equity research firm Fundstrat Global Advisors, who has known Mr. Hwang for roughly three decades. “But he’s doing it in a very unassuming, humble, non-boastful way.”

But in his investing approach, he embraced risk and his firm ran afoul of regulators. In 2008, Tiger Asia lost money when the investment bank Lehman Brothers filed for bankruptcy at the peak of the financial crisis. The next year, Hong Kong regulators accused the fund of using confidential information it had received to trade some Chinese stocks.

In 2012, Mr. Hwang reached a civil settlement with U.S. securities regulators in a separate insider trading investigation and was fined $44 million. That same year, Tiger Asia pleaded guilty to federal insider-trading charges in the same investigation and returned money to its investors. Mr. Hwang was barred from managing public money for at least five years. Regulators formally lifted the ban last year.

ViacomCBS announced plans to sell new shares to the public, a deal it hoped would generate $3 billion in new cash to fund its strategic plans. Morgan Stanley was running the deal. As bankers canvassed the investor community, they were counting on Mr. Hwang to be the anchor investor who would buy at least $300 million of the shares, four people involved with the offering said.

But sometime between the deal’s announcement and its completion that Wednesday morning, Mr. Hwang changed plans. The reasons aren’t entirely clear, but RLX, the Chinese e-cigarette company, and GSX, the education company, had both spiraled in Asian markets around the same time. His decision caused the ViacomCBS fund-raising effort to end with $2.65 billion in new capital, significantly short of the original target.

ViacomCBS executives hadn’t known of Mr. Hwang’s enormous influence on the company’s share price, nor that he had canceled plans to invest in the share offering, until after it was completed, two people close to ViacomCBS said. They were frustrated to hear of it, the people said. At the same time, investors who had received larger-than-expected stakes in the new share offering and had seen it fall short, were selling the stock, driving its price down even further. (Morgan Stanley declined to comment.)

By Thursday, March 25, Archegos was in critical condition. ViacomCBS’s plummeting stock price was setting off “margin calls,” or demands for additional cash or assets, from its prime brokers that the firm couldn’t fully meet. Hoping to buy time, Archegos called a meeting with its lenders, asking for patience as it unloaded assets quietly, a person close to the firm said.