The Former Age
From Halopedia, the Halo wiki
The Former Age is a period in Covenant timekeeping, referring to the era of Sangheili civilisation prior to the formation of the Covenant empire in 850 BCE. The era contained the rise of the Sangheili civilisation including the era of Sangheili interstellar colonialism that culminated with them encountering the San'Shyuum Reformists in a conflict on the frontier world of Ulgethon - kickstarting the War of Beginnings and ending with the signing of the Writ of Union in 850 BCE.
The period seems to have some overlap with the Age of Abandonment, as detailed in the Covenant Progression of the Ages. However, the only known details of the Age of Abandonment refer exclusively to the War of Wills fought between the San'Shyuum Stoics and Reformists on their homeworld, possibly indicating that the Former Age is a term only intended to refer to the pre-Covenant era of Sangheili civilisation. It is similarly unclear how much of the Former Age overlaps with the Age of Seven Worlds in pre-Covenant Sangheili history.
The retroactively-named Sangheili "Vestige" class of spacecraft were initially created in the Former Age and were used to fight the San'Shyuum in space warfare. These vessels include the Lursu-pattern brigantine, R'sikosh-pattern man o' war and Rwaru-pattern carrack.
- Halo Encyclopedia (2022 edition), page 270-271