The purpose of the study is to learn whether an investigational drug called nirogacestat can improve the effectiveness of another drug, elranatamab, in treating patients with multiple myeloma who have had their disease relapse. Elranatamab is comprised of two antibodies (sometimes called a 'bispecific antibody'). Elranatamab binds to T cells (a type of immune system cell) and myeloma cells. This process hopefully causes the T cells to recognize and kill the myeloma cells. Elranatamab is administered as a shot under the skin. Nirogacestat is a type of drug known as a gamma secretase inhibitor and is designed to reduce the shedding of the protein (BCMA) that elranatamab binds to. By potentially keeping more of this BCMA protein on the myeloma cell, it is hoped that the efficacy of elranatamab will be increased. Nirogacestat is taken as a pill.