The UNSC Ultimatum, later known as the CRF Ultimatum, FR Ultimatum, and simply the Ultimatum, was a Halberd-class destroyer built soon after the beginning of the Human-Covenant War. Near the end of the war, it was a member of Taskforce Angel. It served the UNSC for the last time shortly after the Raid of Falaknuma, where it was captured by supporters of Arthur Banks.
As the Ultimatum was preparing to leave the system, a distress signal from a nearby freighter was received and the Ultimatum went to help. The crew of the Ultimatum were able to rescue the survivors of the damaged freighter. As the UNSC believed that they managed to stop the rebellion on Falaknuma, they were taken by surprise as the rescued crew, who were actually supporters of Banks, took control of the ship and jumped out of the system.
After the Ultimatum was captured, it went under many modifications in hiding using parts scrapped from other ships.