The Serpent That Sleeps

An old story that is sometimes told in occult circles is one of an ancient serpent that wrapped itself round a piece of Leviathan's flesh in ages past, bonding with it, and then being encased in amber. This happened in a way that the serpent lost its flesh & cast of its scales when it died. At the time tha amber of a world tree was flung as a meteorite to Earth melting & engulfing both which was then dragged into a chamber of an ancient pyramid made of obsidian & treated with a thin layer of Leviathan's flesh. There it stands with its scales spread around it in this chamber at the heart of the pyramid, soaking in the power of the structure & yet not truly dead. Both the Flesh & the Amber sustains its spirit there, and the evil of them beat like a heart of darkness in this ill-lit room waiting for the time is right to re-awaken.
Mentioned by a select few this is one of the oldest such tales, and always as a counter point to the Star Gods & the Undead cults. The former seems to have minimal link to them & at times little interest overall, and the Undead cults are seen as both allies & enemies.

Summary

The black dragon known as The Serpent Whose Breath Poisons the Land and also Leviathan, made contact eventually with the actual Leviathan in a dimly remembered era. Tasked with continuing the legacy of Leviathan on Earth the dragon continued in its quest of tormenting the region around the Mediterranean bring doom to all it could.
After a few centuries, and after an extensive volcanic blast that severely damaged the Minoan Empire, it was alerted to the Black Pyramid when a fissure was opened. This allowed for the dragon to pierce the heart of evil that spilled into that region of Earth from the outer darkness. Leviathan was pleased after a fashion that its new plaything could access such a power. A place such as this was governed by a triarchy of unbound darkness, chiefly the Void itself that attracted so much energy and allowed it to pass through & pulsate outwards in many different dimensions. The primordial version of the Abyss and the chaotic emotional energies of the Empyrean flowed through touching the Earth and linking it to other Hellscapes.
What caused the volcanic episode seemed to be natural in origin, just the tectonic plates shifting and a build-up of pressure in the volcano itself. What some speculate is that some of the gods from the Empyrean had seen the chance to sow pure chaotic energy by introducing horrific deities & creatures into the world at the same time and hastened the process.
Whatever the details, the region of the black pyramid had already been in existence long enough to attract evil beings for millions of years prior using a system of Underdark volcanic tunnels already present in the area. They didn't know that the place was created millions of years ago by the Old Ones as a repository of knowledge and used the Void as a stepping-stone to & from the Far Realms.
Every story concerning the Serpent That Sleeps is that it found its way into a blasted landscape lit with dull light piercing through breaks in storm clouds. All around are jutting black rocks of basalt, granite and obsidian. Pathways made of the ground down stone litter the landscape and little colour is seen as if leeched out of existence. Sharp carved obelisks jut out at irregular angles scatter the surface and the dominant feature is a massive pure black obsidian pyramid floating slightly above the surface in the distance. Exactly how to get in it is hard to find and likely involves magic.
The Serpent did and explored both it and other hidden structures dotted around the environment. The pyramid is the largest of these structures, which draws the most negative energy into the region with the aid of the obelisks. Small changes had taken place at some point by the original Leviathan and on the inside a thin film of his flesh is regularly found along with brass etchings in a number of exotic languages & sigils. The Serpent cast ritual after ritual in the central chamber of the pyramid allowing forbidden creatures to appear of which it made pacts with. Leviathan itself showed it the way to make small doors to the Labyrinth using all manner of puzzles so only the worthiest of 'sacrifices' made it through to his domain. Other rooms were used to train the darkest of magical arts to those who sought out the tutelage of both the Serpent and vilest mages & clerics. Yet more was turned into treasure rooms, libraries, barracks and storage rooms.
After a time, the Serpent began to tire and seen that sometime in the future that dragons would barely exist anymore, and some of the lesser races would come to steal its secrets. Devising a plan with an unnamed red dragon, and a white dragon far to the north that when the time came the black dragon could be resurrected or returned in some fashion, along with the others. A black dragon scale from its body was taken along with some blood from the red dragon that was nearing the same fate (as in tiring of existence). All of this was combined with the frozen breath of the white dragon. What they done was fashion a spell into the scale drawing on sigils using the blood of the red dragon, and frozen in place by the white dragon.
When completed, each would eat a gemstone of a similar colour to each (white pearl, black pearl, red ruby) and regurgitate it after 24 hours of meditation to allow them to soak in their respective magical energy. At the same time this imprinted a portion of themselves on them. When the black scale was worn in place of a forehead scale on the head of one of them with a scale from one of the remaining dragons exchanged in a similar manner, it would show a pathway to the centre of the pyramid and take over or awaken the Serpent. 
Shortly after the red dragon and the white dragon took their leave with the red dragon carrying the black scale. The white dragon was the youngest and likely still alive. The black dragon eventually started to sleep longer and longer until it passed. At that point it had took a portion of Leviathan's flesh wrapping itself around it. The dragon's body decayed at an alarming rate with the skeleton and scales being the only things left. The scales lie around the room, while the skeleton is still wrapped around the Flesh. 
At about the same time as the spirit left the Serpent it was deliberately trapped in the Flesh and a rogue piece of amber from the great world tree was taken in by the Serpent's servants and purged of its inhabiting god with spells cast on it to preserve the remains. To this day it still stands there awaiting the reawakening of the Serpent by the use of powerful rituals.

THE SERPENT ITSELF
The Serpent That Sleeps was an ancient black dragon that was known to the early Yuan-Ti, who worshipped her as She Whose Breath Which Poisons the Land. She was millennia old at least by that stage if not older still. How many children she had or even her original name is unknown, and due to her age, would regularly disappear to sleep away years or even decades at a time. An expert spellcaster she regularly carried out spells & spell rituals, trucked with demons, devils, jinn, and other horrific creatures, all for the sake of more knowledge & power. Her lairs & treasures were hidden all over what is now the Sahara and surrounding regions. Some scholars believe that she was the reason the Sahara started its desertification was because of her.
Always looking to increase her magical might, led to her travelling all over the Mediterranean Sea, and into both the Aegean & Black Sea too. On more than one occasion she noticed the stirring of something far larger and more powerful than herself in these seas. Seeking out and applying her skills, the Serpent made a connection to an even older more ancient Wyrm dating back to the creation of the world. The creature she encountered was Leviathan, a close relative of the land-based Behemoth and sea9faring Jormungand. Intrigued further as to why she had never encountered Leviathan before she sought to make contact.
Leviathan saw a possible kinship in the Serpent and possibly a way to help consolidate his power in the Hells while having someone take over his mantle. At this point he explained that he had been trapped in a limbo of sorts between the physical world and the Hellscape known as the Labyrinth, created by YHWH to prevent him from taking over and destroying the world. With his mouth being a gateway to Hell, he reasoned that he could turn himself inward and go to the Labyrinth and avoid the gaze of Heaven while the Serpent could carry out tasks for him on Earth.
With this, he would impart some of his knowledge directly into the Serpent and set a few goals increase her own power as she saw fit. Any creature that she slew, tempted, captured or corrupted would serve her in life or undeath but the soul or spiritual energy he could harness & use. That way it was mutually beneficial, and he acted as a mentor showing her plenty of the basics, she needed to serve both her own selfish desires and help expand his own Hell domain.
So far this has helped up till she became encased in the Amber from the Great World Tree, but still, he uses her as a beacon for those with the selfish desires to dominate or explore the darker side of life & death.

Historical Basis

The destruction of the Minoan civilisation and corresponding Atlantis myth was spread to prevent all but those in the know to cover this up. A major volcanic record did happen in the region. Also, the Old Testament / Torah / Koran tells of a massive serpent-like creature called Leviathan in the sea and is classed as a sign of ill0omen.

Spread

The occult community has heard stories of the Black Pyramid for millennia though little has been mentioned of the fate of the Serpent Who Sleeps. Only a few of the vilest texts makes mention of it, and certain groups such as the Yuan-Ti the Children of the Nazarene, and the Necrocult to name a few has any exact idea what lies down there. Most want to seize the magic or study there, and nearly all would happily destroy the Serpent to gain control of the Black Pyramid.

Variations & Mutation

Surprisingly there is little if any variation in the main legend of the Serpent Who Sleeps. If anything, it is the finer details of it that seems to vary. These range in the other dragon types & numbers, the specific spells / rituals used and the like that vary.

Cultural Reception

Suppression outside the cults and occultist circles happens regularly, and even then, it is considered apocrypha with no true verification as to the Serpent Who Sleeps actually existing, and neither the treasure nor library contents ever being discussed openly. Most think that only the exceptionally gifted in the dark arts could ever venture in and survive long enough to return with knowledge. Those that do are treated as insane at best or driven off / killed at worst. Those that do seem to have knowledge generally do not speak of having an 'audience' with the Serpent Who Sleeps.

"The dreaded Serpent That Sleeps. That and the cursed Black Pyramid the only things you will find down there in the Underdark. Sitting at the centre of a massive spiderweb of passages and regions best left unexplored. Though other such places, creatures, and the opposite even, exist there, I would never go there.
Long ago she poisoned so many, corrupted so many and slew so many that all that remains are faded stories of how hate filled towards life she was. Her 'toys' are still unearthed from out-of-the-way places or used for new 'toys' that she's never far away. At least in a spiritual way. Her master, Leviathan, uses her a conduit to release his own powers over the land, and many suspect others do too. As much as slave to all that is unholy and corrupt, as she is a corruptor herself, nothing except the purest can stand against her might.
I cannot go with you and if you do, I hope you turn back before encountering her. May whatever deity you follow protect you and step in before she is finished with you."
Rabbi Omer Abulafia, of the Synagogue of Jerusalem.

Date of First Recording

Unknown but rumoured to be millions of years old.