A Study to Assess Change in Disease Activity and Adverse Events (AE)s in Adult Participants With Immunoglobulin Light Chain (AL) Amyloidosis Receiving Etentamig (ABBV-383) as an Intravenous (IV) Infusion

Inclusion Criteria: * Diagnosis of primary systemic immunoglobulin light chain (AL) amyloidosis. * Eastern Cooperative Oncology Group (ECOG) performance status of \<= 2. * Have at least 1 organ historically impacted by AL amyloidosis. * Considered AL amyloidosis cardiac risk stage 1, 2, or 3a, or considered risk stage 3b with stable cardiac function and markers for 3 months prior to dosing, and have measurable disease of AL amyloidosis as defined by difference between involved and uninvolved free light chains (dFLC) \>= 50 mg/L or meeting high-risk dFLC progression criteria after immediate prior line of therapy. * Has previously been exposed to a proteasome inhibitor (PI) and an anti-CD38 monoclonal antibody. Exclusion Criteria: * Known history of clinically significant (per investigator's judgment) drug or alcohol abuse within the last 6 months. * Known allergic reaction, significant sensitivity, or intolerance to constituents of the study treatment (and excipients) and/or other products in the same class. * Participant has the following conditions: * Other non-AL amyloid disease; * Previous or current diagnosis of symptomatic multiple myeloma (MM), including the presence of lytic bone disease, plasmacytomas, \>= 60% plasma cells in the bone marrow, or hypercalcemia (defined as corrected calcium \> 11 mg/dL); * Active plasma cell leukemia (i.e., either 20% of peripheral white blood cells or \> 2.0 × 109/L circulating plasma cells by standard differential); * Waldenström's macroglobulinemia; * Acute diffuse infiltrative pneumopathy; * Major surgery within 28 days prior first dose or planned during study participation; * History of organ transplant requiring continued use of immunosuppressants; * Acute infections within 14 days prior first dose requiring parenteral therapy (antibiotic, antifungal, or antiviral); * Participant has received an autologous stem cell transplant (SCT) within 12 weeks or an allogeneic SCT within 1 year of the first dose of study treatments.