- "Some were simply beautiful. Art for art's sake. Others were... trophies. Still others were unique scientific curiosities. All secreted in the Mysterium, in the heart of the capital, all untouched, preserved simply for the having of them, of collecting, of adding to the glory of all it meant to be a Forerunner."
- — Splendid-Dust-of-Ancient-Suns describing the Mysterium.
The Mysterium was a Forerunner collection of artifacts located on the equatorial ring of Maethrillian. An entrance to the Mysterium was located beneath the Councilors’ chamber adjoining the council amphitheater. Thousands of items were gathered from the edges of the Forerunner ecumene, brought to Capital, and stored within the Mysterium. Some artifacts were simply art pieces, while others were scientific curiosities. A select few members of the Ecumene Council knew what the artifacts’ purposes were, while most did not. Among the artifacts stored there was Abaddon, a Precursor construct that controlled the Domain and the origin of the legend of the "Organon". Over a century after the Great Purification, the IsoDidact and a group of Forerunners journeyed to the Mysterium to repair the Domain, which had been destroyed in the firing of the Halo Array.