Precursors

The Precursors are an ancient race of aliens that once inhabited the Milky Way galaxy. Their origins are unknown, and they are most likely the only race in the known universe to have achieved Tier 0. What happened to their race remains a mystery.

Legends
"'They were the First—guardians of the Mantle of Responsibility, but their time in this universe is fleeting. The Great Journey waits for no one' — Book of Sorrows, Chapter X"In the beginning, there was nothing. Time did not exist and the universe that came to be was smaller than the pointed end of a needle. Yet, this great emptiness was not entirely empty, for the first visages of life soon appeared. These were the Precursors, and their existence gave birth to the universe.

They were once the undisputed masters of the cosmos, capable of shaping the aether to their will. In fact, it is stated in the Book of Sorrows that the aether was their creation which once served as their ultimate domain. As the universe expanded, the Precursors procured the forces that we know today as gravity. Hundreds of billions of galaxies were formed, but one galaxy, in particular, would outshine them all—the Milky Way galaxy, a stellar body so unique that it formed independently from the Precursors. Billions of years passed, and soon the galaxy was teeming with life.

Eventually, they came to accept that the stability of the aether had begun to crumble due to the combined emotional energies of all life in the universe, so they settled in realspace. The Precursors had intervened very little in the development of the galaxy, and when they made their presence open to all life in the cosmos, they were revered as Gods.

It was once said that hubris is the ultimate enemy of any race. In their arrogance, the Precursors embraced their newfound title as gods, but their reign will not last. No one truly knows why the Precursors mysteriously vanished, but the Book of Sorrows chronicled an epic war between the Precursors and the 'Flood'. The Precursors were driven to a small number of individuals, and their eternal empire crumbled around them. Additionally, the book also mentions the creation of the warp relays and how they were created by the Precursors to navigate across the universe faster.

The universe is in no shortage of mysteries. Whatever happened to the Precursors after their calamitous 'War in Heaven' with the Reapers remains unknown, but the lost remnants of their civilisation could be found in all corners of the galaxy, from the ruined facilities on Mandalore to the lumbering warp relays that remain dormant to this day, waiting for the time of ending, when the universe repeats the endless cycle of death and rebirth.