From the introduction:
Many are the monolithic boulders and chunks of rocky debris that tumble in orbit among the floating castles of the Tomb of the Thaalud, but not all are as they seem. For one of these careening islands of stone is actually the Plague Tree, the home of a marauding band of undead harpies and their master.
“The Plague Tree” is a Dungeons & Dragons adventure suitable for four 15th-level characters. This adventure works with the Forgotten Realms adventure Anauroch: The Sundering of the World [user note: actually refers to Anauroch: The Empire of Shade] and can be used after tactical encounter T4 when used in conjunction with that adventure. Dungeon Masters can also use this side adventure in their existing campaigns after tailoring it for them.