From Halopedia, the Halo wiki
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.