Protector of the Ecumene (Earth-2042)

Protector of the Ecumene, also referred to as commander, was the rank of the supreme commander of the military forces of the Forerunner Ecumene. For twelve thousand years, the Ur-Didact held this post until he was forced to go into a Cryptum. After the Didact's exile, Master Builder Faber assumed his rank. During the Forerunner-Flood War, the IsoDidact assumed the position.

Source

 * https://www.halopedia.org/Protector_of_the_ecumene