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. This is a study to determine the adverse events, change in disease activity, and pharmacokinetics of Etentamig in adult participants with MM. Etentamig is an investigational drug being developed for the treatment of MM. This study is broken into 2 phases; phase 2 with 3 study arms and phase 3 with 2 study arms. Participants in phase 2 will receive 1 of 3 doses of etentamig in combination with daratumumab. Participants in phase 3 will receive etentamig at RP3D in combination with daratumumab, or daratumumab, lenalidomide, and dexamethasone (DRd). Around 660 adult participants with MM will be enrolled at approximately 155 sites worldwide Participants in phase 2 will receive 1 of 3 doses of etentamig as intravenous (IV) infusions, combination with subcutaneous (SC) injections of daratumumab. Participants in phase 3 will receive RP3D doses of etentamig as IV infusions, combination with SC injections of daratumumab, or SC injections of daratumumab, capsules of lenalidomide, and tablet/ IV injections of dexamethasone (DRd). The study duration is approximately 16 years. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and questionnaires.