21B NIXIE LAIR. The entrance to the nixie lair is about six feet below the lake surface. It is obscured by reeds and weeds and it is 95% unlikely to be discovered by casual inspection.
Just within the cave entrance there is a membrane-like structure which is easily passable by a small amount of force. On the outside is normal water, but within the nixie lair there is a strange liquid which is breathable by air-breathers and water-breathers alike. The nixies share their lair with abundant small innocuous lake life: shrimp, crabs, fish, and molluscs.
There are numerous pools within the nixie lair. Each has a different size, color, and properties but the liquid of each pool is unbreathable and distinct from the liquid media that makes up the atmosphere of the lair. The liquid in each pool is separated from the atmosphere of the lair by a bulging dome-like membrane, also easily penetrable. Each pool provides dim radiance (of the same color as the pool) to the area near it, making traversing the lair a wonderland of competing lights, like a rave party underwater. The nixies dislike bright light, but the intensity of the pools' lights is below their threshold for discomfort. Recall also that each nixie will be colored a bright primary or secondary color from drinking the lake water and that also adds to the surrealness of the place.
If the nixies discover intruders or if they have already encountered adventurers as a random encounter, they will attempt to use their charm power to enslave party members. If successful, they will give water breathing to charmed victims and force them to fight the merrows at Area 21D. Note that the charm power of the nixies is much stronger than the charm person spell employed by magic users. The monstrous charm power of the nixies can compel total obedience.
Normally nixies keep enslaved characters for one year, but if the adventurers eliminate the merrows for them, the nixies will be likely to allow charmed individuals free before a year has passed.