**Study Overview:** This study looks at how safe and tolerable a drug called *cemsidomide* is when taken with another drug, *elranatamab*, for people with a blood cancer called multiple myeloma. *Multiple myeloma* is a cancer that affects plasma cells, a type of white blood cell that fights infections. The study will first test different amounts (doses) of cemsidomide to find the safest dose when used with elranatamab. Once the safest dose is found, more patients will be added to check how well the treatment works against the cancer. **Key Points:** - **Duration & Dosage:** Cemsidomide is taken by mouth for 14 days, then paused for 14 days (making a 28-day cycle). Elranatamab is given by injection twice a month. - **Eligibility:** Participants should have been diagnosed with multiple myeloma and have undergone specific prior treatments. They should not have certain other diseases or have had a recent stem cell transplant. - **Compensation & Visits:** Study participants may receive compensation. The study requires regular visits for treatment and monitoring. **Important Note:** Patients with trouble swallowing pills or with certain infections might not be eligible.