The following criteria is provided for health care professionals.
Inclusion Criteria:
* Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1
* Life expectancy of at least 12 weeks
* Documented diagnosis of MM according to the IMWG diagnostic criteria
* Evidence of progressive disease based on Investigator's determination of response by IMWG criteria on or after last dosing regimen
* Measurable disease
* AEs from prior anti-cancer therapy resolved to Grade ≤ 1,
* Adequate organ functions
Exclusion Criteria:
* Pregnant or breastfeeding or intending to become pregnant during the study or within 3 months after the last dose of study drug
* Plasma cell leukemia with circulating plasma cell count ≥ 5% or \>500/microliter (µL)
* Participants with current amyloidosis
* Participants with myelodysplastic syndrome
* Prior treatment with monoclonal antibody (mAb) and antibody-drug conjugate within 4 weeks or 5 half-lives of the drug, whichever is shorter
* Prior anti-cancer therapy (chemotherapy, small molecule/tyrosine kinase inhibitors, radiotherapy) within 14 days prior to first forimtamig administration
* Prior solid organ transplantation
* Active auto-immune disease or flare within 6 months prior to start of study treatment
* Known or suspected chronic active Epstein-Barr virus (EBV) infection
* Hepatitis B virus (HBV) infection
* Acute or chronic hepatitis C virus (HCV) infection
* Known history of HIV seropositivity
* Live vaccine(s) within one month prior to start of the treatment
* Participants not fully vaccinated for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) as per local recommendations
* Previous refractoriness to carfilzomib
* Participants who discontinued prior carfilzomib treatment due to treatment-related toxicity
* Participants with known liver cirrhosis