Grell Nest
Grells are highly dangerous due to their multiple paralyzing attacks. PC priests should have Remove Paralysis spells stacked up for use, but if the players can't figure that out for themselves, be merciless and let them learn from experience. What's more, the old patriarch of the hive - the only reason why the Deepspawn hasn't dared to consume all the Grell for supper by now - is a rarity, a spelluser of power and altogether a formidable enemy. The PCs have their work cut out here. The Grell are intelligent, and after a first raid they take precautions against any further attacks.
The caverns and passages here have ceiling heights of 21 to 30 (20+ d10) feet. They are lit by small iridescent nodules of a luminous mold cultivated by the Grell for that purpose; these are scattered at irregular intervals over all passage and cavern walls. Note that many specific areas here are dangerous because of the razor rock hazard (see DM Reference Card 3). Since Grell fly, this doesn't bother them at all - indeed, it makes an excellent defense against intrusion.
When the PCs are some 100 yards from the entry point to the Grell caverns, a successful Intelligence check allows them to see that a clear scratch mark has been made on a passage wall, pointing diagonally downwards. A small fragment of paper hangs from a rock at the spot the arrow points to, as if some message had been left here and later removed. The PCs may find the missing message in area 9, if they get that far!