Chapter 5 describes myconid community of Neverlight Grove.
The place is one of the quietest and safest of the Underdark, as the few chapter's pages demonstrate.
PCs can take only one side-quest there but they needn't to accomplish it.
Myconids are an alien race, totally different from those common in the Realms, but oddly enough PCs will communicate without problems with them, because Myconids' telepathy can overcome tongue's problems.
Neverlight Grove however, like all other places visited by PCs, shows signs of Demons' Madness, which can be discovered by some investigation.
Hidden beneath the surface of a quiet and peaceful place, Zuggtmoy's evil has taken roots and is constantly growing.
PCs can get aware of it with some area exploration, and they can even face (luckily or unluckily) The Queen Demon of Fungi herself.
Zuggtmoy has CR 23. A real clash with PCs, who will be around 5th or 6th level, can only end with a TPK. Beware.
After foraging or having discovered the truth about Neverlight Grove, PCs will leave the myconid colony, still looking for a way to escape from the Underdark.
How they can you'll see in the next chapter.