The Sidhe (Pronounced Shee) are the elven people of Brython, who are further split into the Aes Sidhe and the Bane Sidhe.
The Aes Sidhe live above ground and often within the deep magical forests of Brython, they are considered to be etheral and mystical peoples with strong links to astral magics. During the Frost-Heim the Aes used their powerful magics to transport their great stone spired dwellings into astral pockets, and their homes now flit between the material and astral planes.
The Bane Sidhe were the elves of the underhill, and lived in great magically created caverns deep beneath the earth. During the scourge they used their magics to transport their people into specially crafted pockets within the elemental plane of earth; an action which damaged their people and sent many of them mad. This fear and insanity, is has been suggested, acted as a beacon to the horrors and nearly every encounter with Bane Sidhe has ended poorly.
The courts of the Aes and Bane Sidhe are now known as the the Faerie Courts and the Court Beneath The Hill respectively.
Both groups of Sidhe took other namegivers under their protection during the Frost-heim, but those that accepted (in both cases) came upon fates they could not have expected.
A final third group of Sidhe align themselves to neither court, having rode out the scourge with other namegivers. The Sidhe courts refer to these Sidhe as wild elves, and they are often found as scouts, archers and woodsmen without fealty to the other courts.