Elder Leviathan

Elder Leviathans are the oldest, strongest, smartest, and strangest of the Leviathans. Other leviathans may be sentient and able to exert their will upon the world, but none can act on such a vast scale as these creatures. They are arbitrary, cruel, detached from reality and common mortal experience; their agendas are mysterious and often arbitrary. They can present as 'people' when necessary, and several are able to mimic mortal socialization perfectly, but do not fall into the mistaken belief that their minds work as ours do.
Most elder leviathans have hosts of warlocks, lesser leviathan thralls who obey their will, and one or more places of communion - residences that they have Hallowed and made their own. They also have corners of the astral sea that their minds reside in, waiting for the wandering minds of could-be warlocks. Each also has its own interpretation of the Masked One's message, that The Strong Survive, which they channel in their own strange ways.
By most metrics, there are fifteen "true" elder leviathans. There could be others, but if so they are well-hidden in the deepest depths and are unknown even to other leviathans. First, you've got your friendly eight, those known for their "benevolence" (neutrality):

• The Lurking Eyes, the most friendly of all warlocks patrons. It is a hoarder, a collector of knowledge and people and treasures, who has nested in Halcyon's Pyramids. It prefers warlocks who are driven by curiosity, and will feed it fascinating shreds of knowledge, fetch it unusual treasures, or who will abduct interesting creatures for it. Its aesthetic is grasping tentacles, drifting eyes, and many whispering voices. Patron of Cosmic Lord Brambleshanks
• The Scarred One, a survival instinct given form. It is patron of those who have fallen out of society and would exist purely for existence's sake, as well as the paranoid and the desperate. It moves its warlocks to seek survival, to protect it (however it sees necessary), and to remind the world of the terror against death that all possess. It most commonly lurks around Ibith, Sonev, and Ekraht. Its aesthetic is excessive life, overgrown existence, and scars
• The Tideweaver, the sphinx of the sea and master of the elements, is the most intellectual and personable of the leviathans. It sees "survival" as a matter of legacy, rather than bodily survival, and "strength" as intellectual or mental. It is the patron of thinkers, riddlers, philosophers, demagogues, and eccentric occultists - it will support any idea as long as the warlock feeds it and grows its glory. The Tideweaver tends to lurk in the equatorial regions, such as Izekra, Makal, West Sonev, and South Desmia. Its aesthetic is water, thought, illumination, and occult mystery.
• The Whispering Thief, or the Whisperer is the puppetmaster of the Leviathans, who revels in the power it has over mortals and the petty dramas they get into. It loves to quietly stoke conflict, watching and guiding its pieces through its many warlocks. Whether it has any goals or simply enjoys watching people destroy themselves and those around them is anyone's guess. Its warlocks can be messy individuals who stir drama wherever they go, or warlocks who are manipulators and observers themselves. It tends to lurk around Maradia, East Ekraht, the Antarctic. Its aesthetic is darkness, the uncanny valley, and any symbols of domination or control it can find - it likes scaring people.
• The Yearning Mind is the keeper of emotion, who sees true life as only happening when you experience intense emotion (and true strength to be strong emotions). It craves to see people go wild, to let their hidden desires and impulses spill into the world. It loves to inflict "madness" upon those it finds - not a mental illness, but a magical state of intense emotional release and low impulse control. It is the patron of hedonists, of subversives, of wild individuals who refuse to follow social norms. It tends to lurk around Suneka, Ukaram, and East Desmia. It revels in whatever aesthetics it feels will unnerve, challenge, and provoke.
• The Howling Melody believes in strength of will and personality as true strength. It hoards art, dabbles in bardism, and curates a vast horde of sycophants who dance around it in an endless parade. It is the patron of those minds who reject reality and demand the world conform to it - to individualistic fellow travelers who prove themselves and their will (or artistic prowess) in some kind of trial. It tends to lurk around East Nafena and across the Great Sea. Its aesthetics are showy, dramatic, flashy, and fiery.
• The Deep King believes that strength can be measured in the number of followers one has, and that the individual can become immortal through what they build. They like governing, being worshipped, making cults, all that. They have cults of every shape and size, and their warlocks tend to either be cultists or cult-makers; loyal servants or vassal kings. It can be found in West Sonev and Larazel. Its aesthetics are vast, mysterious, demanding palaces of endless space and void, like a Leviathan who has become a God.
• The Crowned Hermit is a rowdy leviathan with a past: many centuries ago, it tried to conquer all other Leviathans and subjugate them under itself. It failed, and now it rarely talks with aquatic creatures outside of its domain. It is the patron of those who reject the outside world to seek only what is within themselves - those who seek total self-sufficiency, those who chase nostalgic memories and impossible pasts, and those who seek total mastery of the self. It can be found in the waters along the coast of Inahng and North Nafena. Its aesthetic is tarnished glory and heavens fallen beneath the waves.

Then you've got your seven malevolent Elder Leviathans (who are just more aggressive than the prior eight):

• Apex, the classic leviathan ascended. It values power, it sees the strong as entitled to (perhaps given a duty to) exploit and then slaughter those weaker creatures. All relationships are contests of power, all beings are self-centered, all life can seek is the strength to conquer. Its warlocks are either desperate creatures attracted to its certainty, or jealous monsters who feel the same way. It lurks West of Samvara, in the great sea. Its aesthetic is carnage, meat, hunting, and teeth
• Neverseen, is the rarest and least-known of the elders. It hides in the great sea, keeping its body far from mortals and the other Leviathans. It only extends its mind in search of secrets. It hates being known, and feels power in holding knowledge that others feel should not be known either. It is the patron of those who also cling to the unknown, who would gather it secrets. It tends to be classified as malevolent for its vicious pursuit of those who would know that it even has warlocks - it can be quite violent when made to feel known. Its aesthetic is the unseen, the silent, the empty black.
• The Vile Mass is an alien beast touched by Ederstone, truly unknowable in its aims and horrifying in its power. It is totally inhuman, unable to speak traditionally and detached from all morality. Its goals are arbitrary, and its warlocks are those either drawn to its alien nature or those who feel a kinship with its total isolation from the world. It is near Stildane at most times, though it can wander. Its aesthetics are incoherent, shifting, and dreamlike
• The Frigid Hunger is a creature of want and obsession - it pursues its desires with little care for the future, and it detests the very idea of moderation. It is also the penultimate cannibal, that physically consumes parts of its subordinates as a way to mark its domination over them. Its warlocks have a startling tendency to feel pangs of cannibalistic hunger of their own, though only some embrace this aspect of the Hunger. Most warlocks of the Frigid Hunger generally have some kind of forbidden obsession of their own that they indulge through their patron. The physical form of the Hunger can be found around the Arctic Circle. Its aesthetics are always personal.
• Elder Night is a cryptic creature of ritual, anxiety, and despair. It sees the surface world as the eternal Other doomed to try and destroy the sea, and it interprets all Leviathan impulses as either ritualistic commands from the Sea or as the inevitable call of war against the surface. It is a peculiar creature, prone to taboos and easily angered by things being out of place. It is not entirely irrational, nor is it entirely tyrannical; it demands peace between its servants, for example, and despises unnecessary infighting among the leviathans. But it also lives in its own world of fantasy, and invites those who join it to do so as well. It lives in the waters around Ukaram, Maradia, and South Suneka. Its aesthetics are space and the arcane.
• Old Skybane was always a pious leviathan devoted to worshipping the Masked One, even before it became a ghost. Now, it is a machine of ethereal destruction in service to an unknown force. Its old name has been destroyed (possibly by the Neverseen, but that's just rumor) to prevent exorcism; it has left its old norms and values behind for an eternity of blind, absolute devotion. Its discipline has allowed it to become a powerful magician, and its followers are expected to have a similarly disciplined blind devotion. It can be found near Garadel and West Larazel. Its aesthetic is the eternal storm, ghosts, and icons of death.
• The Shackled God is a creature eternally seeking new kinds of strength and power. It loves experimentation, but it cares little for things that cannot add to its abilities or give it power over some other thing. It is the patron of either power-hungry cultists or power-hungry individualists who feel shackled by society. It craves new kinds of magic, new technologies, new lore, new servants; it is unlimited ego. It is also stuck in a glacier in the antarctic, and is too stubborn to get itself out. Its aesthetic is vastness, coldness, and chains.

The division between neutrality and malevolence is a bit arbitrary, to be honest - all of these Leviathans are capable of helping people and capable of causing harm, and none of them possess a moral compass. Their territories are also only loosely defined, as they are always on the move and always shifting territories.
Not all Elder Leviathans reveal themselves immediately - many choose to hide their personalities and intent from their warlocks until the time is right. Some pretend to be others; their games are complicated.

Individual Articles:

• The Scarred One
• The Vile Mass
• Apex
• Lurking Eyes
• Old Skybane
• Whisperer
• Tideweaver
• Elder Night
• Frigid Hunger
• Shackled God
• Neverseen
• Crowned Hermit
• Deep King
• Yearning Mind
• Howling Melody