A Study of Venetoclax and ABBV-838 Combination Therapy With Dexamethasone in Participants With Multiple Myeloma Whose Cancer Has Come Back or Had No Response to Recent Cancer Treatment

* Eastern Cooperative Oncology Group (ECOG) performance status less than or equal to 1 for participants in the dose escalation portion of the study and ECOG less than or equal to 2 in the dose expansion portion. * Received at least 2 prior therapies including an Immunomodulatory Thalidomide Derivative Compounds (IMiD) and a proteasome inhibitor. * Documented relapsed or progressive multiple myeloma on or after any regimen or is refractory to the most recent line of therapy. * Received at least 2 prior therapies including an IMiD and a proteasome inhibitor. * Documented relapsed or progressive multiple myeloma on or after any regimen or is refractory to the most recent line of therapy. * Eligible for and agree to bone marrow (BM) aspirate prior to treatment start and at designated times per protocol. * Measurable disease at Screening, defined as at least one of the following M component in serum (greater than or equal to 0.5 g/dL) and/or urine (greater than or equal to 0.2 g excreted in a 24 hour collection sample) or serum free light chain greater than or equal to 100 mg/dL with an abnormal κ/λ ratio of less than 0.26 or greater than 1.65.

* Received any anti-myeloma therapy (other than monoclonal antibodies), including chemotherapy, radiotherapy, biological, immunotherapy or an investigational therapy, including targeted small molecule agents within 5 half-lives (or 14 days if half-live unknown) prior to first dose of first dose of venetoclax, ABBV-838, and dexamethasone. * Received anti-myeloma monoclonal antibodies within 6 weeks prior to first dose of venetoclax, ABBV-838, and dexamethasone. * Has a significant history of renal, neurologic (peripheral neuropathy), psychiatric, endocrinologic (diabetes mellitus), metabolic, immunologic, cardiovascular, pulmonary or hepatic disease within the last 6 months. * Received corticosteroid therapy at a dose equivalent to greater than or equal to 4 mg/day of dexamethasone within 3 weeks prior to first dose.