From the front of book:
Two years ago the city of Myth Drannor was overrun with vermin. While adventurers were able to purge the infestation, bigger problems at the time ensured the event was quickly forgotten. Now, an awfully familiar fungus has returned. You and your companions must enter the Citadel of Fungi, a place so toxic nobody has approached it in living memory, to eliminate the source of this corruption before it is too late. A Living Forgotten Realms special adventure set in Myth Drannor for characters of the Paragon tier (levels 11-20).
This adventure is loosely connected with SPEC3-1 Roots of Corruption: Infestation, and draws on events first chronicled during SPEC1-3 Ghost of the Past: Hive of Corruption.