From Halopedia, the Halo wiki
A councilor was a member of the Forerunner Council. The Council was composed of five hundred councilors from various rates. During their service, which typically lasted for a thousand years, councilors led fairly ascetic lives and took vows of abstinence and austerity. Councilors met in the Council amphitheater, a "floating bowl"-like structure connected to the Ecumene Capital. The most senior councilors thought it was disgraceful to mingle with their younger peers. To avoid this during Council proceedings, they would be inserted into the Council amphitheater through a lift tube. The highest position attainable by a councilor was First Councilor.
After the Master Builder Faber used Installation 07 to quell a rebellion that resulted in the deaths of a majority of the remaining San'Shyuum, a revolutionary party of younger councilors opposed to the Master Builder took command. A majority of the Council was replaced by new, mostly younger, councilors that were appointed through bevets. Most councilors of the "Old Council" were killed during the Battle of the Capital when Mendicant Bias fired a Halo at the Capital. Councilors of the New Council were largely killed at the Flood's attack on the greater Ark.
Splendid Dust of Ancient Suns was a Forerunner councilor.
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