Phantom Wasteland
Phantom Wasteland is the primary world area for a Seeker's Rest story on N'Erud.
Ascension Spire
Seeker's Key
Completing the Seeker's Rest story quest requires three Seeker's Keys. You found the first when you entered N'Erud at Seeker's Rest.
The second is somewhere in the Phantom Wasteland. Its exact location is random, but it is always in a seated colossus just like the one you found at Seeker's Rest:
That statue is always in a chamber just past the Phantom Wasteland mini boss.