To Evaluate Safety, Tolerability, and Clinical Activity of the Antibody-drug Conjugate, GSK2857916 Administered in Combination With Lenalidomide Plus Dexamethasone (Arm A), or in Combination With Bortezomib Plus Dexamethasone (Arm B) in Participants With Relapsed/Refractory Multiple Myeloma (RRMM)

Inclusion Criteria

• Capable of giving signed informed consent.
• Male or female, 18 years or older (at the time consent is obtained).
• Have confirmed diagnosis of Multiple Myeloma (MM) as defined by the IMWG.
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1 for Arm A and 0 to 2 for Arm B.
• Have undergone stem cell transplant (SCT), or are considered transplant ineligible.
• Have been previously treated with at least 1 prior line of MM therapy, and must have documented disease progression during or after their most recent therapy.
• Must have at least ONE aspect of measurable disease, defined as one the following: Urine M-protein excretion >=200 milligram (mg)/24 hours, or; Serum M-protein concentration >=0.5 gram (g)/deciliter (dL) (>=5.0 g/Liter), or; Serum free light chain (FLC) assay: involved FLC level >=10 mg/dL (>=100 mg/L) and an abnormal serum FLC ratio ( 1.65).
• Participants with a history of autologous SCT, are eligible for study participation provided the following eligibility criteria are met: Autologous SCT was >100 days prior to study enrollment; No active bacterial, viral, or fungal infection(s) present; Participant meets the remainder of the eligibility criteria.
• All prior treatment-related toxicities (defined by National Cancer Institute Common Toxicity Criteria for Adverse Events [NCI-CTCAE], Version 4.03, 2010) must be Grade <= 1 at the time of enrollment, except for alopecia. Participants with Grade 2 neuropathy can be enrolled into Len/Dex treatment arm, but not into Bor/Dex treatment arm.
• Adequate organ system functions as defined by the laboratory assessments.
• The contraceptions used by female participants be consistent with local regulations, regarding methods of contraception for those participating in clinical studies. 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 child bearing potential (WOCBP) or Is a WOCBP and using a contraceptive method that is highly effective (with a failure rate of <1% per year), preferably with low user dependency, during the intervention period and for at least 4 months after the last dose of belantamab mafodotin and agrees not to donate eggs (ova, oocytes) for the purpose of reproduction during this period. The investigator is responsible for review of medical history, menstrual history, and recent sexual activity to decrease the risk for inclusion of a woman with an early undetected pregnancy.

WOCBP Participants Assigned to Arm A:

• Due to lenalidomide being a thalidomide analogue with risk for embryo-fetal 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 lenalidomide, during therapy, during dose interruptions and continuing for 4 weeks following discontinuation of lenalidomide treatment. Thereafter, WOCBP participants must use a contraceptive method that is highly effective (with a failure rate of <1% per year) for a further 3 months, and agree not to donate eggs (ova, oocytes) for the purpose of reproduction during this period: Two negative pregnancy tests must be obtained prior to initiating lenalidomide therapy. The first test should be performed within 10-14 days and the second test within 24 hours prior to prescribing lenalidomide therapy.

WOCBP Participants Assigned to Arm B

• WOCBP assigned to Arm B must have a negative highly sensitive serum pregnancy test within 72 hours of dosing on C1D1 and agree to use effective contraception during the study and for 4 months after the last dose of belantamab mafodotin or 7 months from the last dose of bortezomib, whichever is longer.
• Male participants using contraception should be c