"We are Forerunners, 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."

Name: Forerunners (Latin: Primoris prognatus, meaning "Firstborn")
Homeplanet: Ghibalb (Heavily irradiated and inhabitable due to stellar engineering accident)
Sentience: Sentient

Strengths:
Immense scientific knowledge in every field.
World builders: able to manufacture structures on immense scales from planet-sized superstructures to structures large enough in diameter to encompass Earth's orbit around the sun. Notable constructions include but are not limited to Halo rings and Shield Worlds like Haven.
Technological achievements include being able to manipulate gravitational forces, create AI's with full sentience, fabricate super-dense materials, ability to create life and create worlds.
Their armour eliminates the necessity for nutrition and extends their lifespans to many millennia.
Able to forge hardlight constructs.

Weaknesses:
For all their power and wisdom, the Forerunners stubbornly held onto the Mantle of Responsibility- the belief in an imperial peace in which the entire galaxy and its inhabitants are under their protection and their protection alone. The Forerunners were also subject to corruption from within. During the final millennia of their rule, the corrupt became the most powerful which lead to them being inadequately prepared for the all-consuming, gestalt parasite which eventually lead to their downfall. Ultimately it was due to corruption and stubbornness that the civilisation fell.

Appearance:

Forerunner appearances were dependant on their Rates (social hierarchy, see culture section). Descriptions are in order of highest to lowest in terms of status:
Builders: Tall and slender. Wore the more ostentatious style of armour to reflect their elevated position.
Miners: Shorter and stockier than most other Forerunner rates.
Lifeworkers: Graceful and tall. Armour enabled them to float.
Juridicials: Unknown.
Warrior-Servants: Powerfully built and robust. Muscular and tall.
Engineers: Unknown.
Theoreticals: Unknown.

Culture:

The Forerunner civilisation was based around the Mantle of Responsibility: the belief that it was their task alone to guard the galaxy and its life forms. As a result of this, they largely secluded themselves above other species. Occasionally they would intervene with such species in ways that would ensure peace but also not rise to challenge Forerunner supremacy. They would even hinder the development of other species to prevent potential uprisings, such as after their war with Ancient Humans after which the species were stripped of their technology and reduced back to the stone age.

The Forerunner societal structure was based upon Rates: a set of formally defined caste-like social strata. These were determined by which caste a Forerunner was born into. Physical alteration would follow soon enough which would ensure that the individuals would suit their societal role (hence the differences in appearance as described above). Young Forerunners who hadn't undergone these processes of mutation and were still in training were known as Manipulars.

Maethrillian, also known as the Capital, was the political center of the Forerunner Ecumene (empire). Maethrillian was a massive structure on which several hundred thousand Forerunners lived. The megastructure also housed the Forerunner Ecumene council; the main governing body of the ecumene.



History:

Although they themselves are essentially extinct, evidence of their existence remained, spread across the galaxy in the form of numerous creations, installations and artifacts.
The Forerunners took upon themselves the role of caretakers of the galaxy, and believed that this "Mantle" had been passed down to them by the Precursors, an even more powerful race that preceded the Forerunner civilization.
Before their disappearance from the galaxy, the Forerunners would name humanity their successors, identifying them with the title "Reclaimers".
In modern history, the alien alliance known as the Covenant revered the Forerunners as gods, deriving much of their own technology from Forerunner artifacts found throughout the galaxy, and held that any who defaced these relics were heretics.
The Forerunners reigned for millennia as the ascendant civilization in the galaxy until the activation of the Halo Array, their Pyrrhic solution to halt the Flood in 97,445 BCE.

Overpowered.

But dead.

Approved