Based on data from the ongoing ABBV-838 monotherapy study (Study M14-467) Arm B dose escalation may be conducted, if deemed necessary. If conducted, Arm B dose excalation will investigate up to 3 doses of ABBV-838 at either weekly (Q1W) or bi-weekly (Q2W) dosing intervals in combination with venetoclax and dexamethasone. Arm B dose expansion portion will investigate either the ABBV-838 Q1W or Q2W dosing interval in combination with venetoclax and dexamethasone at the RPTD combination defined from the Dose Escalation portion.