They appear as large sperm whale sporting an array of teeth more akin to that of a killer whale, their upper half near black and their underside a pale white. Their eyes are larger and more expressive than their more passive sperm whale cousins.
Their physical threat is not their only hazard, for ancient members of that species are highly intelligent and some can perform magic with their songs. These exceptional individuals sometimes establish kingdoms beneath the waves, a moving court consisting of younger Leviathans, killer whales, and even dolphins. These extended pods prey upon other whales, some going so far as to enslave entire pods or to extract tribute from them.
A typical ‘kingdom’ is ruled by 2 or 3 magic capable Leviathans, with 10-20 other leviathans of various ages and genders acting as nobles. Orcas number in the dozens, typically a pod for each senior ‘lord’ and a similar number of dolphins that act as scouts.
These kingdoms generally stay away from major shipping lanes, for while powerful, the Leviathan’s normally see little profit in warring with man. The treasures of man hold little pull with the Leviathans.
The Black Leviathan’s concern themselves primarily with food and family, though those with their horizons broadened by magic tend to have wider interests, some not entirely wholesome.
Should whaling on the scale of 18th/19th century earth occur, this of course will change the picture dramatically, and spur all-out war between the Leviathan kingdoms and humanity.
Humanity is largely unaware of their existence, again due to the caution that the Black Leviathans have towards them. They warred once before with a long-gone surface race, and that racial memory makes them loath to make contact.
The Songs of the Sea
Leviathan magic concerns itself with control of weather, finding (and controlling) of food, secrets and legends, and magic affecting the self for various ends. Infernal magic and necromancy are studied, while Fire Elemental magic is avoided.
Leviathan sorcerers can create looping ‘songs’ that self-sustain and act as recordings. However, they collapse upon being listened to and understood. Leviathan custom is such that those that hear such a song must replace it themselves. Thus knowledge is spread. They will aggressively prevent beings unable to live up to this custom from hearing them. The process of creating a looped song is difficult, more so since these Songs are generally left in painfully deep waters - even for them - to protect them.
Noteworthy Black Leviathans:
The Shark Master
Spurning the normal cetacean hatred of all things shark, this Black Leviathan has delved deeply into the darker verses of the song of the sea and consorts with the evil-shark-people, gaining the servitude of shoals of sharks. Unlike other pods, he has few other whales and no dolphins, having fully thrown his lot in with the sharks. His two lieutenants, non-spellcasting behemoths, are his muscle and are large enough to kill any of his his sharks with a single bite.
The Shark Master and his ‘pod’ would serve admirably as the guardian for some McGuffin.
This Leviathan has an unfortunate curiosity for the works of man, and only a passing concern for man himself. He collects shipwrecks, sometimes causing them himself and is building a semi-floating palace in a hidden island atoll. His pod respects him, but still considers this obsession to be an oddity. In creating his collection, he has a kings ransom of treasure in the sands below his palace.
Crab-folk have moved into the palace, which pleases the Shipwright, giving semblance of life to his creation. It does, however, complicate the process of retrieving any treasure from these waters.
The Black Leviathans have diverse interests and behaviors, and as such can serve a number of roles, not all antagonistic. They could find something of value in human goods, either as plunder or trade, they could know rituals of interest to the landlubbers, or the location of lost ship or city deep below the waves. Perhaps they could lend their services to pirates or would-be emperors.
More details on the real-world inspiration can be found here: