From Halopedia, the Halo wiki
After Faber-of-Will-and-Might was stripped of his title as Master Builder for his crimes against the Mantle of Responsibility, the Ecumene Council was reconstituted from the "Old Council" to the "New Council". A revolutionary party of younger councilors opposed to Faber and his allied Builders took command, with the young, first-form Splendid-Dust-of-Ancient-Suns being made First Councilor.
Splendid Dust led Faber's subsequent trial, though the proceedings were interrupted when the rampant Mendicant Bias led a Flood attack on Maethrillian, ultimately resulting in the firing of Installation 07. Splendid Dust was one of the few councilors to survive the Fate of Maethrillian. With the fall of the ecumene during the Forerunner's war with the Flood, the office was abolished. Splendid Dust, having survived the war, no longer held the title of First Councilor, though still remained a prominent figure among the surviving Forerunners.
A century after the Great Purification, Bornstellar reviewed a message from the Librarian in which she revealed the truth about the Domain and requested that he work to repair it. When Bornstellar consulted with Splendid Dust, the former First Councilor revealed his knowledge of the Mysterium and a device there that could potentially fix the Domain. Amongst his former First Councilor outfit, Splendid Dust still possessed the keys needed to reach the Mysterium and the deadbolt key needed for the device.
Powers and responsibilities
The First Councilor served as the presiding politician over the Ecumene Council, the ecumene's governing body based in the capital of Maethrillian. The metarch-level ancilla that coordinated the trillions of ancilla in Maethrillian only answered to the First Councilor. The First Councilor also served as the First Observer of the Court, the head of the Supreme Mantle Court. The robes of the First Councilor were highly decorated with a collar, trappings, and regalia.
As the head of government of the ecumene, the First Councilor was educated on various matters and information most Forerunners were unaware of, most of this knowledge coming from the Domain. The First Councilor, along with a handful of other councilors, were the only individuals that were aware of the existence of the Mysterium, a collection of rare artifacts and treasures located within the heart of Maethrillian. The First Councilor wore the deadbolt required to access the Mysterium within their robes.
List of appearances