The Sovyrian Stone is actually an enormous crystal from which the legendary portal arch was created. This gate leads to the fabled elven homeland of Sovyrian. Though the portal is well known to all within Kyonin, it is well hidden somewhere in the capital city of Iadara. Few have ever laid eyes upon it. 
The true history of the Sovyrian Stone has been lost to the ages. Legends speak of the mythical elven hero, Candlaron the Sculptor, who created the Sovyrian Stone and many other stone arches that the elves use to this day. These stone arches are known as Elf Gates and together form the Aiudara Network throughout the elven homeland of Kyonin. These gates, although thousands of years old, are mere copies of far more ancient magical gates that exist on Golarion.
After the exodus of the elves from Golarion via the Sovyrian Stone, the exiled demon Treerazer attempted to wrest control of the stone in 2632 AR. Apparently, he attempted to refocus the link away from Sovyrian and forge a connection to the Abyss instead. This prompted the return of the elves and Treerazer's plans were foiled as a result.