Ancient Temple
From UnderlightWiki
The Ancient Temple is a place within the Lost Sea that is comprised of several rooms.
It has the characteristics of a stronghold, yet it is not.
It is similar to Commons Hall, Neutral Home, Fayd'sFortress.
It is not a plane, it resides with the Lost Sea.
Currently, some rooms are infested with Shamblix, so be alert when traveling there.