In the spring of 2007, as the financial markets roared, David J. Grais, a journeyman New York trial lawyer, was searching for new cases to bring. “I wanted to find something very complex that could face big losses,” said Mr. Grais, who had just left a large corporate law firm to hang out his own shingle. ... More than three years and billions of dollars in investor losses later, Mr. Grais is among a handful of lawyers at the center of the latest chapter of the mortgage mess. Mr. Grais was back at the Core Club last month, this time behind a lectern, trying to persuade about 100 investors in depressed mortgage-backed securities to fight the banks that issued them.