Endurance-of-Will

 was a Forerunner Warrior-Servant of the Promethean rank. During the Human-Forerunner war, she served as adjutant to the great Promethean commander: Bitterness-of-the-Vanquished, and during the Forerunner-Flood war she was the Ur-Didact's second-in-command.

Early life and career
It is unknown when Endurance-of-Will was born, although she is likely close in age to the Ur-Didact and the Librarian. Before the marriage of the Didact and the Librarian, Endurance was the Didact's lover. After the Didact's marriage, Endurance became adjutant to Bitterness-of-the-Vanquished, an ancient Promethean commander who was at the time the greatest tactician in Forerunner history. She held this position throughout the Human-Forerunner war and possibly through the Kradal conflicts, though this is unconfirmed.

Forerunner-Flood War
- During the Forerunner-Flood war, Endurance served as the newly returned Ur-Didact's chief lieutenant, stationed on Requiem. It is unknown to what extent she filled this capacity. As the war drew to a desperate close, the Ur-Didact stole a Composer used it to digitize his entire force of Prometheans into non-organic combat A.I.s. Endurance oversaw the creation of these creatures with some misgiving, even as the Ur-Didact traveled to Omega Halo and composed the entire human population there. The Librarian, outraged at this abomination, traveled to Requiem without the Ur-Didact's knowledge, even as the Iso-Didact traveled to the Lesser Ark to activate the rings. Once on the Shield World, she found and confronted Endurance, convincing her to help put a stop to the Ur-Didact's madness. The Librarian then incapacitated her husband and sealed him inside his ship's Combat Cryptum. Endurance then safeguarded Requiem for an unknown amount of time. Being inside the Shield World, she was safe from the firing of the rings. Her fate is unknown.