This is an open-label, multicenter clinical trial designed to evaluate the safety and potential efficacy of venetoclax and ABBV-838 combination therapy with dexamethasone in participants with relapsed or refractory multiple myeloma (MM) who have received 2 or more prior lines of therapy for multiple myeloma (MM). The study will consist of 2 arms: Arm A and Arm B (if applicable). Each arm will have a dose escalation and dose expansion portion.