Aboleth

Malevolent aberrations resembling eels or prehistoric jawless fish, these psychic monstrosities are incredibly intelligent despite their appearance, and must constantly remain covered in a slimy secretion in order to breathe. If air-breathing creatures are caught in this mucus, they will transform into mindless aquatic slaves known as skum. Aboleths are biologically immortal, and will only die if they are killed by another creature. They are a relatively newly-developed species, seeing as the first mention of their existence came in modern fantasy literature.