Photo of Jillian Mariutti “There was no surprise that Libor as we knew it would have to change,” Mariutti says.

NEW YORK CITY—Libor, an index that underpins $350 trillion in financial contracts, has been a benchmark for global borrowing for nearly half a century. But it will soon go the way of BOAC and the Beatles, two British institutions that were still recent history when Libor was devised. Will the sunset of Libor in 2021 plunge lenders and borrowers into darkness and confusion? And what will take its place?