Forest of Two Kingdoms (2008)

A One-Round Living Greyhawk Keoland Regional Adventure

From the introduction:

A thousand years ago, fleeing the destruction of the Twin Cataclysms, twelve Suel tribes migrated east through what became known as Slerotin's Pass. Three remained in the Sheldomar Valley: the Rhola and their Lizhal allies, the Neheli and their Linth allies, the Lerara... and their Malhel masters. A thousand years in the Far Realms may have altered the physical form of the Malhel, but their motive remains the same; it's time to Rejoin the Empire.

The finale of the Forest series for [characters level] 8 - 16.