A Study Evaluating the Safety and Pharmacokinetics of Escalating Doses of Forimtamig in Participants With Relapsed or Refractory Multiple Myeloma (r/r MM)

Inclusion Criteria:

* Previously diagnosed with Multiple Myeloma (MM) based on standard criteria.
* Dose Escalation Phase and Dose Expansion Phase: Participants with r/r MM who have previously received therapy with an Immunomodulatory drug (IMiD) and Proteasome Inhibitor (PI) and are intolerant to or have no other option for standard-of-care treatment according to the Investigator.
* Life expectancy of at least 12 weeks.
* Agreement to provide protocol-specific biopsy material.
* AEs from prior anti-cancer therapy resolved to Grade =\<1.
* Measurable disease.
* For female participants of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse), use contraceptive measures and refrain from donating eggs.
* For male participants: agreement to remain abstinent (refrain from heterosexual intercourse), use contraceptive measures and refrain from donating sperm.

Exclusion Criteria:

* Inability to comply with protocol-mandated hospitalization and activities restrictions.
* Pregnant or breastfeeding or intending to become pregnant during the study or within 3 months after last dose of study drug.
* Prior use of any monoclonal antibody, radioimmunoconjugate, or antibody-drug conjugate for MM treatment within 2 weeks before first forimtamig administration.
* Prior treatment with systemic immunotherapeutic agents within 2 weeks before first forimtamig administration.
* Treatment-related, immune-mediated AEs associated with prior immunotherapeutic agents.
* Treatment with radiotherapy, any chemotherapeutic agent, or treatment with any other anti-cancer agent (investigational or otherwise) within 2 weeks, prior to first forimtamig administration. Limited field palliative radiotherapy for bone pain or for soft tissue lesions is allowed.
* Autologous or allogeneic stem cell transplantation (SCT) within 100 days prior to first forimtamig infusion and/or signs of chronic graft versus host disease or ongoing immunosuppressive medication.
* Prior solid organ transplantation.
* Active auto-immune disease or flare within 6 months prior to start of study treatment
* Any medical condition or abnormality in clinical laboratory tests that, in the Investigator's or Medical Monitor's judgment, precludes the participant's safe participation in and completion of the study, or which could affect compliance with the protocol or interpretation of results.