Multiple myeloma (MM) is a cancer of the blood's plasma cells. The cancer is typically found in the bones and bone marrow (the spongy tissue inside of the bones) and can cause bone pain, fractures, infections, weaker bones, and kidney failure. Treatments are available, but MM can come back (relapse) or may not get better (refractory) with treatment. This is a study to determine the safety, efficacy, and pharmacokinetics of Etentamig in adult participants with MM.