Sub-study of Belantamab Mafodotin (GSK2857916) in Combination With Nirogacestat in Participants With RRMM

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 (\<=)2 is due solely to skeletal complications and/or skeletal pain due to MM. * Participants with measurable disease defined as at least one of the following: Serum M-protein greater 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 (\<0.26 or \>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 receiv…