You mustn't play this chapter to finish the campaign but it's useful and contains a lot of hints for further adventures.
You can simply use it to level up your players before the next, and final chapter.
PCs receive a message from one of the myconids they have previously met: Stool, Rumpadump or Basidia, the sovereign of Neverlight Grove.
The myconid gives them news about Zuggtmoy's plan.
The Queen of Fungi has enslaved many creatures with her spores which have spread throughout the Underdark by faerzress.
Now she's leading her slaves to Araumycos, a huge cave which is a single sentient being made of Fungi colonies, the greatest living being of the Underdark and maybe of all Faerun.
Her goal is to merge with Araumycos, binding it to her by spells and therefore negating its will, and then recreate her Abyssal Realm on Faerun.
Basidia gives to the PCs some spores which make them be protected from Zuggtmoy's spells and can help them to connect with Araumycos when near to it.
You'll find a special encounter table for Araumycos cave and surroundings.
Meanwhile Zuggtmoy is uniting with Araumycos by "marrying" it.
PCs should use Basidia's spores to enter into Araumycos's mind and free it from Zuggtmoy's binding.
Araumycos' mind is treated like the Astral Plane, using the same rules of the DMG.
Zuggtmoy eventually discovers them but they are helped by Juiblex, another Demons, because wants to free Araumycos for its own purposes. The King of Oozes want Araumycos for himself and its oozes because they want to feed of it.
After having freed Araumycos PCs are caught in the middle of the battle between Zuggtmoy and Juiblex.
The book insists about Juiblex winning over Zuggtmoy but you can surely change the outcome.
Demons' blood can be used as component for the Dark Heart if the players haven't found Yeenoghu's in chapter 14.
PCs can runaway or fight, killing one of them or both.
If the Demons survive the players will find them again, among all others, in the next and final chapter.