The purpose of this study is to evaluate the safety and effectiveness of an experimental drug called ABBV-383 in people with relapsed or refractory multiple myeloma (multiple myeloma that has come back or did not get better) who have received at least 2 prior lines of therapy. Researchers will evaluate whether treatment with ABBV-383 is safe, well-tolerated, and works better than the current standard available therapies to treat multiple myeloma. A previous study already showed that ABBV-383 is safe and effective for the majority of patients with relapsed multiple myeloma, but it is unknown how it compares to currently approved medications. Participants will be randomly assigned to 1 of 2 groups. One group will receive ABBV-383, and the other will receive standard available therapy.