The study will consist of 2 arms: Arm A and Arm B (if applicable). Arm A dose escalation will investigate up to 3 doses of ABBV-838 at 3-week dosing intervals (Q3W) in combination with venetoclax and dexamethasone. Arm A dose expansion portion will investigate the ABBV-838 Q3W dosing interval with venetoclax and dexamethasone at the recommended phase two dose (RPTD) combination defined from the Dose Escalation portion.