Sub-study of Belantamab Mafodotin (GSK2857916) in Combination With Nirogacestat, Lenalidomide, and Dexamethasone 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 receiving physiological doses oral steroids (\<10 mg/day), inhaled steroids or ophthalmalogical steroids.
* Participants with platelets value for Adequate Organ System Function is ≥75 × 10\^9/L.

Exclusion Criteria:

* Participants with current corneal epithelial disease except mild punctate keratopathy.
* Participants with evidence of cardiovascular risk.
* Participants with known immediate or delayed hypersensitivity reaction or idiosyncrasy to drugs chemically related to belantamab mafodotin or any of the components of the study treatment. History of severe hypersensitivity to other mAb.
* Participants with active infection requiring antibiotic, antiviral, or antifungal treatment.
* Participants with other monoclonal antibodies within 30 days or systemic anti-myeloma therapy within \<14 days.
* Participants with prior radiotherapy within 2 weeks of start of study therapy.
* Participants with prior allogeneic transplant are prohibited.
* Participants who have received prior Chimeric Antigen T cell therapy (CAR-T) therapy with lymphodepletion with chemotherapy within 3 months of screening.
* Participants with any major surgery (other than bone-stabilizing surgery) within the last 30 days.
* Participants with prior treatment with an investigational agent within 14 days or 5 half-lives of receiving the first dose of study drugs, whichever is shorter.
* Participants with \>=grade 3 toxicity considered related to prior check-point inhibitors and that led to treatment discontinuation.
* Participants who have received transfusion of blood products within 2 weeks before the first dose of study drug.
* Participants must not receive live attenuated vaccines within 30 days prior to first dose of study treatment or whilst receiving belantamab mafodotin +- partner agent in any sub-study arm of the platform trial and for at least 70 days following last study treatment.
* Participants with presence of active renal condition (infection, requirement for dialysis or any other condition that could affect participant's safety). Participants with isolated proteinuria resulting from MM.
* Participants with known human immunodeficiency virus (HIV) infection, unless the participant can meet all criteria: a) established anti-retroviral therapy for at least 4 weeks and HIV viral load\<400 copies/milliliter (mL) b) cluster of differentiation 4 plus (CD4+) T-cell (CD4+) counts \>= 350 cells/microliter (µL) c) No history of Acquired immunodeficiency syndrome (AIDS)-defining opportunistic infections within the last 12 months in which case the participant would be eligible for CE Phase only. For participants receiving nirogacestat, HIV drugs that are strong Cytochrome P450 3A4 (CYP3A4) inhibitors are prohibited. HIV drugs that are moderate CYP3A4 inhibitors, while permitted, should be co-administered with caution and must be accompanied by nirogacestat dose modifications.
* Participants with uncontrolled small and/or large intestinal disease.
* Participants with uncontrolled skin disease.
* Participants with any condition causing hypophosphatemia, hypokalemia or hypomagnesemia which is refractory to electrolyte replacement.
* Participants with previous administration of a gamma secretase inhibitor.
* Participants with concomitant administration of a strong CYP3A4 inhibitor or inducer.
* Participants with active or history of venous thromboembolism within the past 3 months.
* Participants with evidence of active mucosal or internal bleeding.
* Participants with contraindications to or are unwilling to undergo protocol-required anti-thrombotic prophylaxis or unable to tolerate antithrombotic prophylaxis.
* Participants who discontinued prior treatment with lenalidomide due to intolerable adverse events.