A Phase III, Open-Label, Randomized Study to Evaluate the Efficacy and Safety of Single Agent Belantamab Mafodotin Compared to Pomalidomide Plus Lowdose Dexamethasone (Pom/Dex) in Participants With Relapsed/Refractory Multiple Myeloma (RRMM) (DREAMM 3)

• Capable of giving signed informed consent.

• Participants must be 18 or older, at the time of signing the informed consent. In Republic of Korea, participants must be over 19 years of age inclusive, at the time of signing informed consent.

• Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2.

• Histologically or cytologically confirmed diagnosis of Multiple myeloma (MM) as defined according to IMWG, and : Has undergone autologous stem cell transplant (SCT), or is considered transplant ineligible; Has received at least 2 prior lines of anti-myeloma treatments, including at least 2 consecutive cycles of both lenalidomide and a proteasome inhibitor (given separately or in combination), and must have documented disease progression on, or within 60 days of, completion of the last treatment or must be non-responsive while on last treatment, where non-responsive is defined as not achieving at least Minimal Response (MR) after 2 complete treatment cycles. In such cases lack of achieving of at least MR must be determined no earlier than at least 4 weeks after the last treatment.

• Has measurable disease with at least one of the following: Serum M-protein \>=0.5 gram per deciliter (g/dL) (\>=5 gram per Liter); Urine M-protein \>=200 mg/24 hours; Serum free light chain (FLC) assay: Involved FLC level \>=10 milligram per deciliter (mg/dL) (\>=100 mg/L) and an abnormal serum FLC ratio (\<0.26 or \>1.65).

• Participants with a history of autologous SCT are eligible for study participation provided the following eligibility criteria are met: Transplant was \>100 days prior to initiating study treatment; No active infection(s).

• Adequate organ system functions as defined: Absolute neutrophil count (ANC) \>=1.0\*10\^9/L; Hemoglobin \>= 8.0 g/dL; Platelets \>= 50x10\^9/L; Total bilirubin \<=1.5\* Upper limit of normal (ULN) (isolated bilirubin \>1.5\*ULN is acceptable if bilirubin is fractionated and direct bilirubin \<35 percent); ALT \<=2.5\*ULN; Estimated glomerular filtration rate (eGFR) \>=30 milliliter per minute per 1.73 square meter (mL/min/1.73 m\^2); Spot urine (albumin/creatinine ratios) \<=500 milligram per gram (mg/g) (56 milligram per millimoles \[mg/mmol\]).

• Contraceptive use by men or women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies. Male participants are eligible to participate if they agree to the following during the intervention period and until 6 months after the last dose of study intervention to allow for clearance of any altered sperm: Refrain from donating sperm PLUS, either: Be abstinent from heterosexual intercourse as their preferred and usual lifestyle (abstinent on a long term and persistent basis) and agree to remain abstinent OR Must agree to use contraception/barrier as detailed below depending on whether they are randomised to Arm 1 (belantamab mafodotin) or Arm 2 (pom/dex), even if they have undergone a successful vasectomy: Agree to use a male condom throughout study treatment including the 6 month follow-up period even if they have undergone a successful vasectomy and a female partner to use an additional highly effective contraceptive method with a failure rate of \<1 percent per year when having sexual intercourse with a pregnant woman or a woman of childbearing potential who is not currently pregnant. Four weeks for male participants on Treatment Arm 2 (pom/dex).

• A female participant is eligible to participate if she is not pregnant or breastfeeding, and at least one of the following conditions applies: Is not a woman of childbearing potential (WOCBP) OR Is a WOCBP and agrees to abide by the following: Arm 1 (belantamab mafodotin): Use a contraceptive method that is highly effective (with a failure rate of \<1 percent per year) which includes abstinence, preferably with low user dependency during the intervention period and for 4 months after the last dose of study treatment. Arm 2 (pom/dex): Due to pomalidomide being a thalidomide analogue with risk for embryofetal toxicity and prescribed under a pregnancy prevention/controlled distribution program, WOCBP participants will be eligible if they commit either to abstain continuously from heterosexual sexual intercourse or to use two methods of reliable birth control (one method that is highly effective), beginning 4 weeks prior to initiating treatment with pomalidomide, during therapy, during dose interruptions and continuing for at least 4 weeks following discontinuation of pomalidomide treatment. Two negative pregnancy tests must be obtained prior to initiating therapy. The first test should be performed within 10-14 days and the second test within 24 hours prior to prescribing pomalidomide therapy. And agrees not to donate eggs (ova, oocytes) for the purpose of reproduction during this period. The investigator should confirm the effectiveness of the contraceptive method(s) ahead of the first dose of study intervention.

• All prior treatment-related toxicities (defined by National Cancer Institute- Common Toxicity Criteria for Adverse Events \[NCI-CTCAE\], version 5.0, 2017) must be \<=Grade 1 at the time of enrollment, except for alopecia and Grade 2 peripheral neuropathy.