Abaseli Magnuse is an Ayleid ruin, in Elsweyr. For thousands of years, it has remained hidden under the sands.
During the time of the Ayleid Dominion, Abaseli Magnuse was the largest and most important temple of Magnus in Elsweyr. It was the home temple of the High Herald of Magnus. Sometime before the fall of the Ayleids, the temple was already in ruins and buried under the sand. History does not record when this happened. By the rise of the First Aldmeri Dominion, the location of Abaseli Magnuse had already been forgotten.
During the height of the Oblivion Crisis, a Golden Saint went rogue and sought Abaseli Magnuse, for his own purposes. Nothing of his motives are known, except that he sought the original copy of the Magnuse Malatu, which he believed was buried in the temple. He succeeded in entering the temple, but was killed by other Dremora before he could steal the book. Everything was left behind, in the ruined temple, except for a Daedra-skin map.
Since this time, the top half of the Skystone, an ornament on top of the temple's highest dome, has been visible above the sand. Since it just looks like a small pyramid with the name of Magnus on it, the locals think it's just a shrine or waystone.
Sir Enkara Venim recently discovered that the Skystone is not just a roof ornament, but also serves as the covering for a doorway, apparently once used for access to the roof. He entered the ruins and encountered the ghost of the High Herald, who asked him to bury the temple under the sand, in return for being allowed to read from the Magnuse Malatu.