[You may want to read the previous post: The Elemental Evil in Mystara – part I: the Scenery ]
WARNING, SPOILER ALERT: the next map is GM-only, so don’t look at it if you are a player!
The adventure can be set either in the AC 1000-9 period or in the following years. The first is the time of the Wrath of the Immortals and of the global war between the largest nations of the Known World. The second is when the Alphatian Empire just sunk, while the Thyatis Empire, in its defeat, is not strong enough to get hold of the old Alphatian Colonies.
In this moment, for the northern coast domains facing Thorin Bay, like Blackrock and Verdan Kingdoms and also the independent cities in the southern coast of Esterhold and Zamara seas (Port Marlin, Eversummer, Rock Harbor and Sinbad’s Gate), start a liberation process that will end with their independence, after the end of the world and Alphatia’s disappearance.
These are the years during which the Egg of Coot, whose conscience has woken up after more than four thousand years, starts to act to get again a physical form to continue in its plan of total conquest from the point it left it a long time ago.
Following the events in the The Return of the Egg – third and last part the Egg of Coot, hit by a huge energy wave, lost its physical form and its conscience became prisoner of a minhir  .
After the planet shift, kilometers of water of the new pole sea submerge the monolith, in which the Egg conscience had found sanctuary.
Today, from its watery grave, and thanks to dreams and visions it can reach the entire world using various monoliths strewn on Mystara, survived to the holocaust. The Egg can thus corrupt the Wu  Benares, convince him to forge four weapons imbued of the Elemental Evil energy, and actually kick off the plan for its large-scale return to Mystara.
The origin of the Elemental Evil cult is deeply rooted in the long gone Evergrun Elven Realm era and in the rebellious endeavors of the being that would later become the Immortal Idris .
The damned elf plays a very important role in the Elemental Evil cult development even though the goddess-to-be does not realize from the beginning the real nature of the same Dark Power, that counseled and moved her actions.
All began when Idris started hearing in her head a flattering voice that leads her with false promises to challenge openly Ordana and to try to destroy the Unique Tree, the goddess’s gift to her beloved people.
After defeating Idris, the furious Ordana curses her to live again and again her pain in a long chain of reincarnations … forever!
After several years, in her first reincarnation, Idris got in touch with the fascinating Blackmoor civilization and created the first cult of the Elder Elemental Egg, never realizing that she contacted the evil Egg of Coot.
Her masterpiece, completed with the help of other corrupt elves, was the Elemental Evil Temple construction, in what are now the Sumber Hills.
When Idris had to flee and found her death, the cult somehow survived her demise.
The newly reincarnated Idris always lost memory of her past lives, as per Ordana’s will, so she had forgotten the cult created by her.
It is also possible that after the Great Rain of Fire and the Jennites’ Invasions, the cult was eradicated from the area and with it all the remembrance of its rites.
In the following years, the ones of the fallout from the nuclear explosions ravaging Mystara surface, some Kogolor dwarven families  under Belsimer clan’s banner arrived in the area, from the Blackmoor territory.
This people, after a lengthy exodus through Jen dry steppes, chased by its warriors and humanoid tribes, reached the Sumber Hills, found the old underground complex and chose to remain there.
Sheepherders and miners, they burrowed in the earth depths nonetheless and they found the old temple built by Idris elves but were unable to clear the dungeons from dangerous creatures and simply collapsed and shut down the old tunnels.
Tyar-Besil Kingdom, from the Belsimer city dwarven name, was little and not so long lived.
Few years after the completion of the constructions in Sumber Hills, trolls and giants came to endanger the old dwarfs dream, forcing them to leave the area.
Few survived the diaspora and just a few lucky ones found sanctuary in the Hollow World.
During the following years, sparse adventurer’s parties and Jennites avant-gardes came to the buried Tyar-Besil fortress and the elven temple underneath.
Unfortunately, no one had the idea of compiling an account of the exploration and so names of places and cities went lost in the meantime.
Only in the years of the alphatian invasion, twelve centuries later, an adventurers’ party, devoted to the Immortal Ixion, by the name of Knight of the Fire Wheel , found the remains of the old dwarven city and started its ruins’ exploration.
They returned many times to Tyar-Besil resolved to eradicate the evil dwelling there and, to this end, they built a fortress on each access point to the old Kogolor city.
They feared the presence of something extremely evil, felt deep under the Sumber Hills and ready to act: to face it they organized a sort of permanent guard on the area.
Borderland stories do not agree on the fate of the Knights of the Fire Wheel party.
The most trustworthy tale claims that after less than fifty years, they were caught and eradicated by the biggest and deepest Jennite incursion ever seen in the area.
For centuries, the towers stood alone and the locals started to call them the Haunted Fortresses, at times empty at times occupied by monsters.
The lost towers and the Kogolor Reign would have completely been forgotten if not for the coming of a mysterious stranger: Benares the Wu.
The cult survived for some centuries, even though it had changed into a very different thing from the religion created by Idris.
Waking up from its millennial sleep, the Elemental Evil Egg became the Elder Elemental Eye and the seat of the cult was renamed the Fane of the Eye.
The memory of its origins and nature long gone from memory, the Egg of Coot saw a perfect opportunity to begin anew its operations in the area in a more quietly and cautious manner.
After a while, it also realized that forging a new physical form for itself required that it tapped directly to the four elements primordial energy.
To this end, it would not need common elemental material but Primordial Elements, only found in very powerful and extremely old elemental creatures, which existed in times before any mortal reached the Elemental Plane and tamed the first elemental: a time when the Elemental Planes were wholly uncontaminated! Then it needed to taint these four Elemental Princes, corrupt their Primordial core with its vile powers, and summon them to the First Material Plane through four passageways called Elemental Nodes, created just to this end.
For this purpose, the Egg corrupted Benares, a Wu from Sanctuary, the legendary city on Sagarmatha Mountains, in the northern part of Skothar continent.
The Wu was chosen because he was quite versed in the magical arts and prone to using Blackmoor era items.
He undertook a very long journey that brought him, through the Jen Steppes and Thonia kingdom, to the Esterhold peninsula and to the Fane of the Eye.
Driven by the Egg’s visions, Benares the Wu built four mighty apocalypse weapon, imbued with the power of the four corrupt elements.
So he forged: the air lance Windvane, the fire dagger Tinderstrike, the water trident Drown and the earth war pick Ironfang.
After the end of this feat Benares left without a trace, leaving behind on the Elemental Evil altar the four apocalypse weapons.
These weapons were soon claimed by the four Elemental Evil Prophets, which came a few months ago, from the far ends of the world, to reinstate the old cult.