The Elder Halls are a central location deep underground within Perfect Worlds such as Freneskae & Gielinor. They contain the sleeping Elder Gods in egg form as seen in Fate of The Gods and an Anima Mundi rift. During the Great Revision the Elder Gods 'hatch' crawl to the rift and then proceed to suck the anima of the world dry to revive themselves and destroy every planet except the one containing the current Elder Hall. It can be deduced that the Elder Gods spend their time and energy creating worlds that can produce the maximum amount of Anima so that they can be nourished. However it is believed from Zaros' account that this is not always accomplished and that many Elder Gods died and or born stillborn from a lack of sufficient Anima thus making the Elder God & the multiverse in danger of ceasing to exist.
=Freneskae
At any point in time there are two Elder Halls in existence, the one in the Perfect world of the last cycle ( And as such the lowest plane of existence) and the current one in which the Elder Gods sleep ( The highest plane). Freneskae being the lowest plane and Gielinor the highest exemplifies this relationship. Freneskae had its anima sucked dry by the Elder Gods, and is a wretched place. A single Elder God remains in the volcano that leads to the halls - Mah. Thought stillborn, she was abandoned by her siblings and brought new life to Freneskae.
Gielinor
At the end of Fate of the Gods, the god Zaros tasked his general and champion, the Mahjarrat Azzanadra to find the Elder Halls of the Elder Gods on Gielinor, as he hopes to rouse them in a bid to save Gielinor from the same fate as Freneskae.