PFS is defined as time from randomization until earliest date of disease progression (PD), determined by Independent Review Committee (IRC), according to the International Myeloma Working Group (IMWG) Response Criteria, or death due to any cause. PD= increase of >=25% from lowest value in >=1 of following (serum M-protein [absolute increase >=0.5 grams per deciliter {g/dL}]; serum M-protein increase >=1g/dL [when lowest M-protein >=5g/dL]; urine M-protein [absolute increase >=200 milligrams per 24 hours {mg/24h}]; participants without measurable serum & urine M-protein levels, difference between involved & uninvolved serum free light chains (sFLC) levels [absolute increase >10mg/dL]; appearance of new lesion,>=50% increase from nadir in Sum of the products of the maximal perpendicular diameters of measured lesions (SPD) of >1 lesion, or >=50% increase in longest diameter of previous lesion >1 centimeter (cm) in short axis.

Intent-to-Treat (ITT) Population included of all randomized participants whether or not randomized treatment was administered.

CRR is defined as percentage of participants with a confirmed complete response or better.

ORR is defined as percentage of participants with a confirmed partial response or better.

CBR is defined as percentage of participants with a confirmed minimal response (MR) or better

DOR is defined as time from first documented evidence of partial response or better until first documented progression or death, whichever occurs first.

TTR is defined as time from the date of randomization and the first documented evidence of response (PR or better) among participants who achieve partial response or better.

TTP is defined as time from the date of randomization until the earliest date of documented date of disease progression or death, whichever occurs first.

OS is defined as time from the date of randomization until the date of death due to any cause.

Progression-free survival on subsequent line of therapy is defined as time from randomization to disease progression after initiation of new anti-myeloma therapy or death from any cause, whichever occurs first. If disease progression after new antimyeloma therapy cannot be measured, a PFS event is defined as the date of discontinuation of new anti-myeloma therapy, or death from any cause, whichever is earlier.

Minimal Residual Disease (MRD) negativity rate is defined as the percentage of participants who are MRD negative status by next generation sequencing (NGS).

An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of a study treatment, whether or not considered related to the study treatment.

Blood samples will be collected for the analysis of hematology parameters.

Blood samples will be collected for the analysis of clinical chemistry parameters.

Urine samples will be collected for the urine dipstick analysis.

Blood samples will be collected for PK analysis of belantamab mafodotin.

Blood samples will be collected for PK analysis of belantamab mafodotin.

Blood samples will be collected for PK analysis of belantamab mafodotin.

Serum samples collected for the analysis of the presence of ADAs using validated immunoassays. All samples will be tested in screening assay, and positive samples will be further characterized for antibody titers.

Serum samples collected for the analysis of the presence of ADAs using validated immunoassays. All samples will be to be further tested in screening assay, and positive samples will be further to be characterized for antibody titers.

The PRO-CTCAE is a patient-reported outcome measure developed to evaluate symptomatic toxicity in participants on cancer clinical trials. The PRO-CTCAE includes an item library of 124 items representing 78 symptomatic toxicities drawn from the CTCAE.

The EORTC QLQ-C30 includes 30-items with single and multi-item scales. These included five functional scales (physical functioning [PF], role functioning [RF], cognitive functioning [CF], emotional functioning [EF] and social functioning [SF]), three symptom scales (fatigue, pain and nausea/vomiting [N/V]), a global health status (GHS)/ Quality-of-Life (QoL) scale, and six single items (constipation, diarrhoea, insomnia, dyspnoea, appetite loss [AL] and financial difficulties [FD]). Response options are 1 to 4. Scores were averaged and transformed to 0 to 100, a high score for functional scales/ GHS/QoL represent better functioning ability or health-related quality-of-life (HRQoL), whereas a high score for symptom scales/ single items represent significant symptomatology. Baseline was defined as latest pre-dose assessment (Day 1) with a non-missing value, including unscheduled visits. Change from Baseline was calculated by subtracting Baseline value from the post-dose visit value.

The EORTC Quality of Life Questionnaire 20-item Multiple Myeloma module (QLQMY20) is a supplement to the QLQ-C30 instrument used in participants with multiple myeloma. For the EORTC IL52, disease symptoms domain of the QLQ-MY20 will be used for bone aches or pain, back pain, hip pain, arm or shoulder pain, chest pain, and pain increasing with activity. The individual component scores in the disease symptom domain are averaged and transformed linearly to a score ranging from 0 to100. A high score for disease symptoms represents a high level of symptomatology or problems. A high score represents a high/healthy level of functioning.