The Forgotten were a race of upright serpents, last seen in the Crystal Desert nearly 250 years ago, as well as in the Realm of Torment during the time of Nightfall. They're devout followers of the Six Human Gods, and are said to be linked to the Facets that reflect the power of the Human Gods., and are servants of Glint, having been the ones who freed her from Kralkatorrik's control.
Human legends state that the Forgotten were originally brought to Tyria by the Six Gods to serve as the world's custodians during the gods' reforming of the planet, though this has been called into question due to the true history of the gods being revealed. Human records also say that the Forgotten arrived on Tyria in 1769 BE, though whether this refers to the world or the continent is unknown.
According to Warden Illyra, the Forgotten predate the earliest known civilizations. Recent discoveries of history, such as jotun stelae, state that they are much older, dating back to the time of the Elder Dragons' last rise. The Forgotten are one of the five sentient races recorded by jotun to have survived the Elder Dragons' last cycle of awakening. During this time, the Forgotten are known to have used magic that could not be corrupted by the Elder Dragons, and were successful in removing the mental connection between Glint, then known as Glaust, and her master Kralkatorrik at the Altar of Glaust in Arah. They were subsequently hidden from the Elder Dragons with the other surviving races (except mursaat) by Glint until the Elder Dragons went to sleep.
Archaeological evidence shows that the Forgotten were spread throughout continental Tyria as well as far as Cantha when humans arrived on the world. They are known to have warred with the charr on the borders of the Blazeridge Mountains sometime in Tyria's past prior to 100 BE until they retreated, but otherwise their stewardship is thought to have been largely peaceful.
Just prior to the war against Abaddon and the Exodus of the Gods, the Forgotten warred with the Margonites in the Crystal Sea. When Abaddon was cast down, the Forgotten became his wardens, and in 174 AE, they retreated into the Crystal Desert after humanity's spread pushed them out from other lands. There, they became custodians of Glint as well as serving as part of the trials of Ascension.
In Turai Ossa's time, the Forgotten were believed to be mindless beasts and attacked by Elonian pilgrimages. This resulted in a war that, along with Elonia's own civil war, wiped those pilgrimages out.
In the Maguuma
Around 200 years ago, the Forgotten appeared in the Maguuma Jungle, where they made preparations to protect Tyria from the rise of the Elder Dragons. Under their guidance, the golden city of Tarir was built as a place to collect and safeguard artifacts of Glint's legacy. Mortals were recruited to become Exalted, who serve as the custodians and protectors of Tarir. After completing their tasks, the Forgotten left Tyria, and Tarir and the Exalted lay dormant until the death of Glint.
- ^ Defining the Lore of Guild Wars 2 - A Q&A with Jeff Grubb, Ten Ton Hammer