The Monsaic Sanctum is located deep within the caves of Monsaic Lines, and is the location where the celebration ceremony at the end of Abe's Oddysee takes place. It is simply a large chamber with a big rock face/pedestal on which Abe stands in the final cutscene. It is said that the bird-portals which carry escaping Mudokons away from RuptureFarms lead to this chamber. The arriving Mudokons then mingle and meet up with lost family members and friends.
It is briefly seen in the beginning of Abe's Exoddus, where Big Face accidentally knocks Abe off the rockface, causing him to have a spiritual vision, in which The Three Weirdos first appear.