A Study to Evaluate Adverse Events, Change in Disease Activity, Tolerability, and How Intravenous ABBV-438 Moves Through the Body in Adult Participants With Multiple Myeloma (MM)

Inclusion Criteria: * Has relapsed or refractory Multiple Myeloma (MM) with documented evidence of progression during or after the participant's last treatment regimen based on the investigator's determination of the standard International Myeloma Working Group (IMWG) (2016) response criteria: * Relapsed defined as previously treated myeloma that progresses and requires initiation of salvage therapy; * Refractory defined as disease that is nonresponsive (failure to achieve minimal response) while on last therapy, or progresses within 60 days of last therapy. * Has measurable disease at screening, defined by at least 1 of the following within 28 days prior to enrollment: * Serum M-protein \>= 0.5 g/dL (\>=5 g/L); OR; * Urine M-protein \>= 200 mg/24 hours; OR; * Involved serum free light chain (sFLC) \>= 10 mg/dL (100mg/L), provided serum FLC ratio is abnormal; * Must have had 3 or more prior lines of therapy with exposure to a proteasome inhibitor (PI), an immunomodulatory imide drugs (IMiD), and an anti-CD38 therapy and are intolerant to, or unable to access, available therapies that are known to confer clinical benefit to participants with relapsed or refractory (R/R) MM. Note: A line of therapy consists of relapsed or refractory 1 complete cycle of a single agent, a regimen consisting of a combination of several drugs, or a planned sequential therapy of various regimens. Exclusion Criteria: * Known history of Central Nervous System involvement by MM. * History of idiopathic pulmonary fibrosis, organizing pneumonia (e.g., bronchiolitis obliterans), drug-induced pneumonitis, or idiopathic pneumonitis.