Sub-study of Belantamab Mafodotin (GSK2857916) in Combination With Nirogacestat, Lenalidomide, and Dexamethasone in Participants With RRMM

The primary purpose is to determine the safety and tolerability of belantamab mafodotin in combination with nirogacestat, lenalidomide, and dexamethasone, and to establish the recommended Phase 2 dose for combination treatment to explore in the cohort expansion (CE) phase in participants with RRMM. This study is a sub study of the Master protocol (NCT04126200).

Inclusion Criteria: * Participant must be 18 years of age inclusive or older, at the time of signing the informed consent. * Participants must have histologically or cytologically confirmed diagnosis of Multiple Myeloma (MM), as defined by the IMWG. * Participants having at least 3 prior lines of prior anti-myeloma treatments including an immunomodulating agent (IMID) a proteasome inhibitor (PI) and an anti-CD38 monoclonal antibody. * Participants with a history of autologous stem cell transplant are eligible for study participation when, transplant was \>100 days prior to study enrolment and with no active infection(s). * Participants with Eastern Cooperative Oncology Group (ECOG) performance status of 0-1, unless ECOG less than equal to (\=)0.5 gram per deciliter (\>=5 gram per liter) or Urine M-protein \>=200 milligrams (mg) per 24 hours or Serum free light chain (FLC) assay: Involved FLC level \>=10 mg per deciliter (\>=100 mg per Liter) and an abnormal serum FLC ratio (\1.65). * Participants who have tested positive for Hepatitis B core antibody (HBcAb) can be enrolled if the following criteria are met: Serology result HBcAb+, Hepatitis B surface antigen (HBsAg)-; HBV deoxyribonucleic acid (DNA) undetectable during screening. * Participants who are currently receiving physiological doses oral steroids (\