Cranium Eels
One of the dominant life forms in the depths of the Deepwell and throughout Sigil's underseas are the cranium eels. They do not appear to be actually related to cranium rats at all, but simply another species that uses similar survival techniques. Like the rats the eels do form gestalt consciousnesses, intelligences, and psionic abilities as they congregate but there the similarities seem to end. The eels do not seem to live in a true hive mind as when they congregate the greater mind formed is never just a single personality, but instead seems to be made up of many personalities sharing the greater whole. As one sage pointed out "The eels will very often speak for themselves using the word "I" and I've never heard of the rats even using that word."
Eels appear similar to normal primeworld freshwater and sea eels with long bodies, pointed snouts and two fins running the length of their bodies. The major difference is a soft sparkling bioluminescence that will ripple down their lengths depending on their moods. Other than that they range in colors and coloration from mottled greys and greens to blacks and even blues, and deep reds. Some have been seen with their bodies marked with tattoos going down their sides. They range in size from tiny ones measuring to just a few inches to gigantic ones that are many yards long. There is a very real possibility that even larger ones lurk in the deepest parts of the waters. Eels grow in intellect and mental abilities as they grow in size. Those of a length that is less than a three feet are no more intelligent than common eels, and are dominated by the larger eels when the congregate. At around three feet the eels begin to develop their individual personalities and points of view and are herded in schools by larger caretaker eels. Eels between three and five feet are almost never seen alone or unaccompanied by larger eels by outsiders; if they are seen at all they are in schools of at least five eels of their size with one eel several feet longer than them. At six feet the eels are more or less "adults" with slightly lower intelligence than the average berk.
Eel society is extremely hierarchical with the smaller eels being dominated by the larger older ones. Unlike their rat "cousins" the eels will regularly speak with outsiders if they are large enough or able to congregate to form telepathic speech and are known to trade for food (live slaves and meats mostly, though fruits have been accepted), the occasional item of magical or psionic power, and information. In return they trade services like bringing up items that have sunken into the deeps, or allowing visitors to dive down into their deep territories, or they trade giving information.
You asked for it so here it is my first species that lives in the deeps. As always please feel free to invent more and please if you do post them.
I thought I'd add a little more about the eels the basic idea of their religion.
Cranium Eels worship the male of a species of fire-fish holding them sacred. Fire-fish males can grow to very large sizes but most have bodies about the size of the average dwarf. The males sport long flowing fins of bold bright colors that glow softly most times and can burst into flaming bright hues of color when the fish is excited, fighting or mating. Males fight each other on sight and these fights are considered by the eels to be sacred events the outcomes of which shape the future. Juvinile and non-dominant males fins are held back with dimmed colors and keep to small schools away from the prime breeding areas. Females are small and dull colored and collect around the large males in harems or form small roving schools. They move off alone to lay eggs and the eels do not consider them especially sacred.