**Study Summary:** This study is looking at how well a combination of two medicines, *cemsidomide* and *dexamethasone*, works for people with a type of blood cancer called multiple myeloma that has come back or didn't respond to treatment. Multiple myeloma is a disease where cancer cells build up in the bone marrow and crowd out healthy blood cells. **Dexamethasone** is a type of medicine that helps reduce inflammation and manage some types of cancer. **Cemsidomide** is a new drug being tested for its effectiveness in treating this condition. **Key Points:** - **Participant Requirements:** Only adults (18+) who have tried at least three different treatments for multiple myeloma can join. They must be in good health otherwise, and women cannot be pregnant. - **Study Length & Visits:** Participants will take medicine and be monitored closely. The exact number of visits isn't specified, but it requires a commitment. - **Risks & Considerations:** Participants cannot have certain other diseases or conditions, and they need to avoid donating blood or sperm during the study and for a period after finishing the treatment. Please note that other rules might apply, and participants will be informed before joining.