Multiple Myeloma (MM) is a cancer of the blood's plasma cells ( blood cell). 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 (relapsed) or may not get better (refractory) with treatment. This is a study to determine adverse events and change in disease symptoms of ABBV-383 in adult participants with relapsed/refractory (R/R) MM. ABBV-383 is an investigational drug being developed for the treatment of R/R Multiple Myeloma (MM). This study is broken into 2 Arms; Arm A (Parts 1 and 2) and Arm B. Arm A includes 2 parts: step-up dose optimization (Part 1) and dose expansion (Part 2). In Part 1, different level of step-up doses are tested followed by the target dose of ABBV-383. In Part 2, the step-up dose identified in Part 1 (Dose A) will be used followed by the target dose A of ABBV-383. In Arm B a flat dose of ABBV-383 will be tested. Around 120 adult participants with relapsed/refractory multiple myeloma will be enrolled at approximately 30 sites across the world. Participants will receive ABBV-383 as an infusion into the vein in 28 day cycles for approximately 3 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.