Gods to some, devils to others. Harbinger of death for all.
Abyss dwellers, they live most of their long life in the depth below the Expanse. They are supposed to be able to go far deeper than any Sphere could ever, belonging to a whole different world. Indeed, they never rise very high, and none has ever been seen anywhere near the sky. Although they seem to age, their lifespan is extremely long, perhaps infinite. The oldest memory still remembers some of them exactly as they are now
We cannot exclude the hypothesis that they have a limited lifespan, but they live on a different timescale than us. We already know skips undergo hundreds of cycles before becoming Sphere, whereas the oldest Oblogg may only witness two such cycles. If the Leviathans live on even larger scales, the Expanse may very well meet its end before they do.
Of the origins
For the Spheres were created as a boon from our all-father, the Leviathans are their counterparts, so that we are not to forget our holy duty. We are but humble krill populating His immensity, and the beasts come for the wrongdoers and the unfaithful beyond redemption.
For the few of them who actively hunt, the Leviathan seems to only come to Spheres or large schools of skips. The size of the prey does not seem to be what motivates them since they have been seen ignoring impressive Jelly blooms twice the size of the Sphere of Oppollln. Rather, it looks like they are natural predators of the Spheres. Not only are they the only beings that can take down such a great creature with ease, but they are also bigger and faster, superior in every way. This raised an interrogation that gained popularity in thinker circles: which came first of the Sphere or the Leviathan? Did the Leviathans simply find in the Sphere a worthy prey or was this empty ecological niche filled out by very natural shenanigans?
The main church of the Father Ocean claims they have been hatched from the shattering of the Hateful Beast in its imprisonment in the Abyss, long before the apparition of life in the Expanse. To the dissident church of the All-Father, they have been made alongside the Sphere to punish those who transgress the law of the divine. The cultists of the Sunken Gods tell they have been around since the very beginning of everything, as they are thought to be the makers of life itself. Rationalist Krevvl, allegedly the most eminent expert on Leviathans, certifies that Spheres predates Leviathans. They are coming from a time before the Apocalypse. Albeit no living Memory remains from this time, she found clues on the existence of Spheres long before this time, but nothing about Leviathans.
When such powerful beings roam the world alongside the meeks, it is no surprise that they came to be considered godly figures. Every religion has a place for them, and a cult even celebrates some of them. The Sunken Gods are a group of Leviathans which left a lasting impression on the cult's founders, elevating them to deities. To them, the other Leviathans are also of divine essence, albeit lesser ones. The Spherical church is of a different opinion, as to the priests of this religion Leviathans are devil incarnates, demons coming from dark and impure waters to prey upon the pure Spheres. This is of course subject to discord, heated discussions, and bloodshed between the two factions.
Death of a divine tyrant
Killing a Leviathan is not an astonishing feat. It is utterly impossible. Even the rare agnostics acknowledge them as being integral parts of the ocean, an embodiment of the water's will. Or so were they considered before the advent of the Seekers. During their journey to the abyss, Anhvall's crew advanced through many hardships, venturing further than anyone ever had, from the heights of the sky to the depth of the Abyss.
One of their numerous feats was coming down The Deep Way Down, but even that achievement pales in comparison to what they did to return from the Abyss. They ascended back the Bladed streams in the hollowed shell of a Leviathan, looking like Hghariogs, a Leviathan of tremendous size. The Revealer himself confirmed later that they fought the beast to the death and triumphed after a harsh fight. Although he was already weakened and suffering from its fight with another Leviathan, it was still an unprecedented feat.
Despite the skeptic voices, the Leviathan was never seen again, and Anhvall would then be famous as the Leviathan slayer. However, they were not greeted with a celebration of their heroism, but the worry of retaliation from its kin and hatred from the Cult of the Sunken Gods, when it was not pure disbelief. They could not stand the idea of one of their cherished god brought low and vowed to be the demise of the Seekers, gathering an alliance of other churches to form the Divine League, an organization set to track and destroy the infamous skip. Most people simply viewed the story as a fairy tale to further establish the Seekers' reputation and considered it as such.
The death of Hghariogs sparked inspiration in a whole generation of young blood trying to prove themselves. If they can be felled, then there is honor and glory to gain. Oblivious to the backfire the Seekers suffered after their feat, a hundred skips roam the Expanse, looking for an enemy worth their remarkable fighting abilities. The god hunters, as they like to call themselves, never succeeded despite the attempts and the thousands of death. Somehow they still find new recruits, and even Spheres are suspected of funding skips in the vain hope that one of them might reach its goal.
On the mind of a giant
Krarrhjkckk is one of the smallest Leviathan, deemed almost manageable by the Leviathan hunters. Slightly bigger than Oppollln, it got an elongated Red body, and tremendous speed. It is the one with the highest count of survivors, mainly because it crosses them too quickly for it to notice the skips or Spheres it just met. One time however, a particularly ambitious group of hunters managed to hook onto the giant. Despite the overwhelming force put on their skip, they managed to get to the top of its head.
There, spheriers landed on the thick scales of their target, trying to connect to his mind. Way harder than it is to do so from inside the body, it is still possible to enter the mind of a Sphere from the outside shell. The adventurers wanted to know if the mind of a Leviathan would be of the same structure as a Sphere. All six were seasoned spheriers, skilled enough to gain the title of master spherier on a modest Sphere. That did not help them with whatever they found there. Not even a breath after they said they found a way in, all of them collapsed, their inside burnt to a crisp by the heat of their brains. The crew managed to escape swiftly, but that only added to the mystery of the mind of the Leviathans. At least it was then clear that it was not something mortal beings could ever dream of coming close to understanding.
Cloud Sculpting is an artform with tight rules, especially on the subjects that can be represented. If most are merely despised and frowned upon, the representation of Leviathans is the only thing that is absolutely forbidden. Once, fools transgressed the taboo, leading to the disaster that is a roaming sculpture up the sky.
On their feeding habits
We all need food of some kind to live. The bigger the being, the more food it needs. Leviathans seem to completely escape this general rule. Realistically, beings of their size would need to eat more in a cycle than the whole Expanse has to offer. However, attacks on Spheres or even skips are so rare we can surmise it has more to do with the poor victims being on the path of the Leviathan than a real hunt. Considering this, Leviathans should be starving and withering out, but they don't seem to care the slightest. This led me to the conclusion that they are either not bound by the laws of the ocean, or that the unknown Abyss holds gigantic amounts of food, enough for them to feed on for their immense lifespan.
On their signature
Every living being emits a signature, an uncontrolled spasm that creates waves around them, effectively declining their identity. They are not unique to individuals, but transmitted through one's lineage, except for the most complex species like Spheres which share only a part of their signature with their offsprings. But even the immense Sphere of Oppollln has a signature that is understandable for the lesser minds. Leviathans do not abide by the rules of our world. They do possess a signature, but it is more of a sinister growl than a real signature. Higher minds can extract from them a sort of signature, just a small part. Far from the real thing, it still gives an idea for the lesser minds unable to sense anything else than a paralyzing howl.
On their display
For the few of us who use eyesight to perceive the far environment, Leviathans come in no more than three hues: pale, translucent light, reddish, or dark, however, the latter two may only be a different perception of Red. As to why a Leviathan would need to conceal itself is up to debate, I believe it to be a remainder of its immature stage when it may be vulnerable to predators. If my theory proves right, the dark ones would be the youngest and paler the beast, the older it is. It would correlate with the fact that there exists far more pale Leviathans than red ones.
To those who make use of other senses to grasp the appearance of things, it gets confusing. Apparently, many Leviathans emit erratic electrical pulses, only perceptible by the most acute species. These pulses, just like their signature, are incoherent and way too massive to handle. Eels and such species often go in hiding when a Leviathan is sighted, as they may turn mad just by feeling the approach of such a titan.