How can one describe it? I cannot, for the one time I saw it, it was so vast I could only see some of it, and that was just the part that had risen from the water. You might as well ask me what the merfolk castles on the bottoms of icebergs look like.
Part whale, part shark, part dragon, part serpent, part elemental, and yet all of these things and none of them at the same time. Bahamoot is an ancient one that defies the title. The beast is young as far as such creatures go, just reaching it's adulthood and strength. It's body is massive, a central fuselage like body with a vaguely shark like streamlined head. The dorsal ridge rises up like craggy mountains, an iridescent mixture of blues and whites with the upper most peaks a metallic purple. The tail, again as long as the body flicks lazily behind, with the mountainous ridge running down its length, ending in the knife like peak of the tailfin. The dorsal fins are thick, with extensions that almost seem like dragon wings, if dragon wings were large enough to hold small mountains and villages in their crooks and ridges.
Bahamoot came within three day's sailing of our coast. The tides turned, and ran in strange directions, no doubt the effect of its great tail and the winds were changed as they deflected off of it's back. In the middle of the rainy season we saw a week without the driving rains. A dozen ships sailed out to explore what had occurred and most came back, telling of churned seas and old wrecks driven from the ocean bottom onto the shores, and of strange creatures disturbed from great depths, bot nothing more. Other saw a great purple mountain range, but that a strange tide pushed them away as the mountains of this unknown island seemed to move away from them. But we dared and came near the head of the great beast. It's eyes were as wide as our ship was long and it had many of them.
A peculiar madness came across our ship and crew. Men began to worship it, and had strange dreams of it, claiming that in their dreams it flew through the heavens, and answered the prayers of pious men and brought ruin to the decadent lands of evil and greedy men.
Ancient Things Were All New Once
Bahamoot is less than a century in this world, and though men are prone to worshipping it, it is not a god or any other sort of divinity. It is likewise not an elemental. As a member of the Ancient Race, Bahamoot is something entirely of this world, but made from much more durable stuff than mundane flesh and bone, though it has both. It is immortal in the fashion of the gods, and it's vast powers are drawn from the sort that elemental powers command. It's concerns are its own, and it thinks very little of man.
The Cult of Bahamoot
There are hundreds of sailors and other seafolk who have seen Bahamoot, and the way the waters eddy in it's passage. These often simple and superstitious folk have formed a cult to venerate the Ancient One. They are easily noticed, as they have adopted purple and white robes and often affect a hat that looks like either a mountain, or a shark fin. The cult is a peasant cult, and has neither arcane magi nor clerics, as Bahamoot doesn't grant cosmic secrets or divine blessings.
Here There Be Dragons
For the most part, Bahamoot is reclusive and seldom ventures near commonly inhabited coasts. It likewise seldom frequents the common trade routes. The ancient one favors the vast doldrums and other calm places in the ocean. It is prone to spending long periods of time passive and resting. Unlike other living creatures, Bahamoot doesn't subsist on either flesh of animals or plant biomatter. The vast bulk of the ancient one is sustained by a mix of natural 'respiration' where the entity consumes the natural energy generated by the planet. It feeds at ley lines, nodes, devil's triangles and other such places. It moves from place to place, feeding on the ambient energy. It also gains sustenance from sunlight, moonlight, and from the oceanic primal force.
This doesn't mean that the ancient one cannot consume, because it can. The ancient one can snack on the other great beasts of the sea, such as great whales, giant sharks, kraken, and even the physical avatars and incarna of celestial, elemental, or infernal beings. While these beings are indeed powerful, they are either visitors/intruders, or are by nature subservient in the case of elementals.
Just Another Big Ass Monster: Bahamoot is another giant monster to fill out the ranks of the aquatic realm. Scale powers and abilities to somewhere between a titan and a terrasque but suited for water. Taking down the ancient one should be the culmination of a campaign and be the joint effort of many fleets of ships and massive amounts of magic and firepower.
Rise of a New Sea God: Move over Dagon and Cthulhu, Bahamoot's comical clergy has suddenly started demonstrating divine magics. The power is raw and unrefined, and is often dangerous to the clerics. These first generation clerics might lack control and finesse, but their ability to cast spells is beyond what it should be. Eventually the ancient one will figure out the proper way to channel power to it's mortal followers.