Japan Expansion Cohort: Evaluation of Efficacy and Safety of Belantamab Mafodotin, Bortezomib and Dexamethasone Versus Daratumumab, Bortezomib and Dexamethasone in Participants With Relapsed/Refractory Multiple Myeloma

Inclusion Criteria: * Confirmed diagnosis of multiple myeloma as defined by the International Myeloma Working Group (IMWG) criteria. * Previously treated with at least 1 prior line of multiple myeloma (MM) therapy and must have documented disease progression during or after their most recent therapy. * Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2. * Must have at least 1 aspect of measurable disease, defined as one of the following; 1. Urine M-protein excretion \>=200 mg per 24-hour, or 2. Serum M-protein concentration \>=0.5 grams per deciliter (g/dL), or 3. Serum free light chain (FLC) assay: involved FLC level \>=10 mg per dL (\>=100 mg per liter) and an abnormal serum free light chain ratio (\<0.26 or \>1.65). * All prior treatment-related toxicities (defined by National Cancer Institute Common Toxicity Criteria for Adverse Events \[NCI-CTCAE\] version 5.0) must be \<=Grade 1 at the time of enrollment, except for alopecia. * Adequate organ function Exclusion Criteria: * Intolerant to daratumumab. * Refractory to daratumumab or any other anti-CD38 therapy (defined as progressive disease during treatment with anti-CD38 therapy, or within 60 days of completing that treatment). * Intolerant to bortezomib, or refractory to bortezomib (defined as progressive disease during treatment with a bortezomib-containing regimen of 1.3 mg/m\^2 twice weekly, or within 60 days of completing that treatment). Note: participants with progressive disease during treatment with a weekly bortezomib regimen are allowed. * Ongoing Grade 2 or higher peripheral neuropathy or neuropathic pain. * Prior treatment with anti-B-cell maturation antigen (anti-BCMA) therapy. * Prior allogenic stem cell transplant. * Any serious and/or unstable pre-existing medical, psychiatric disorder or other conditions, including renal, liver, cardiovascular, or certain prior malignancies. * Corneal epithelial disease