Prothean (Earth-4664)

"We are Protheans, guardians of all that exists. The roots of the galaxy have grown deep under our careful tending. Where there is life, the wisdom of our countless generations has saturated the soil. Our strength is a luminous sun towards which all intelligence blossoms. And the impervious shelter beneath which it has prospered."

- The Ur-Didact, giving a speech on the purpose of the Mantle of Responsibility.

The Protheans (Latin Primoris prognatus, meaning "Firstborn") were an ancient sentient species of technologically advanced beings whose empire—known as the Citadel—encompassed three million fertile worlds in the Milky Way Galaxy. The Protheans reigned for millennia as the ascendant civilization in the galaxy until the activation of the Halo Array, their Pyrrhic solution to halt the Reapers, in 100,000,000 BCE. Although they themselves are essentially extinct, evidence of their existence remained, spread across the galaxy in the form of numerous creations, installations and artifacts, most notably, the Mass Effect Field and the Mass Relays.

The Protheans took upon themselves the role of caretakers of the galaxy, and believed that this "Mantle" had been passed down to them by the Forerunners, an even more powerful sentient species that preceded the Prothean civilization. "Prothean" is a literal translation of the species' name for themselves; they identified themselves as such because they believed that they held an impermanent place in the universe's Living Time, though, in time, they too would be succeeded by other, superior species. Before their extinction from the galaxy, the Protheans would name Humans their successors, identifying them with the title "Reclaimer".

In modern history, the alien hegemony known as the Covenant Empire revered the Protheans as gods, deriving much of their own technology from Prothean artifacts found throughout the galaxy, namely the Mass Effect Field, and held that any who defaced these relics were heretics.

History
"One hundred thousand years ago, the galaxy was populated by a great variety of beings. But one species—eons beyond all the others in both technology and knowledge—achieved dominance. They ruled in peace but met opposition with quick and brutal effectiveness. They were the Protheans—the keepers of the Mantle, the next stage of life in the Universe's Living Time. And then they vanished."

The Protheans were created by the Forerunners, a powerful species responsible for seeding the Milky Way Galaxy with life and creating many other species, fifteen million years ago. The Protheans were seeded on the planet Ghibalb in the Orion Complex. The world served as the hub of the Protheans' fledgling interstellar civilization, comprised of twelve systems within the nebular complex, until it was rendered uninhabitable by an astroengineering disaster. The Protheans had attempted to manipulate several stars in their region of space, but accidentally caused a series of supernovae that resulted in their planet being burned to a cinder by radiation. This event nearly caused the Protheans' extinction.