Harry Potter and the Return of Atlantis
#01—Rising Sea Levels
A group of Muggle archeologists in Antarctica stumble upon a mysterious structure under the ice. After years and extensive digging, they finally reach the structure to find it is an ancient tomb of some long-forgotten ruler of a mysterious society. However, one ignorant but greedy Muggle touched a forbidden magical item of the ancient ruler. The item was a Horcrux of the ruler, which awakened upon contact. The Horcrux took immediate possession of the Muggle's body and subjugated the rest of the team to his will.
The King of Atlantis unleashes his unfathomable power upon the world by causing the oceans to rise. First he melted the ice at the North and South Poles and all existing glaciers. But the worst part was for Wizarding World was that the King's influence on the planet disrupted their abilities to Apparate, the King had created a planet-wide anti-Apparation field to hinder his enemies. The world was caught unprepared for the sudden and unexpected flooding.
Many cities went underwater within a few days, millions were killed or drowned, and the people were at the mercy of the ever-rising tide. The King then called people to his realm. They were only allowed entry if they swore absolute loyalty to him on a binding magical contract. As weeks turned into months and years, more people began submitting to his will and joined his Eternal Empire.
October 30, 1994
Beauxbatons and Durmstrang had just arrived for the Triwizard Tournament when the floodwaters hit. The Durmstrang ship is swept away and destroyed in the flood. Unable to hold back the tsunami, many became trapped inside of Hogwarts when Dumbledore erected powerful new wards to shield the castle and people. With the anti-Apparation wards keeping them inside and the waters flooding vast portions of the country, none were willing to venture outside for help or escape using Portkeys.
Harry and his friends weren't among those trapped. They got caught in the waters and swept away. Had Hermione not been able to transfigure a log into a boat, they and several others would've drowned. Sirius is caught unawares and drowns in the flood as it sweeps over Hogsmeade.
With his power fast returning, King Atlantis then he caused massive earthquakes that released hot underground water tables, which just heated the planet up even faster. The rapid heating of the planet caused waves of tropical storms that further sunk the landscape. In desperation, many millions of people fled their homes and countries. Many of them were inexplicitly drawn to Atlantis.
Scrambling to bring order to the chaos, the sunken Wizarding Worlds of Europe band together to form a new magical government called the Unified Parliament of Magic, led by the Congress of Magical Nations. It is mostly comprised of Purebloods and extremely rich/influential/powerful Muggleborns who are only vying for their power and prestige. The Parliament quickly has a floating city (called Poseidon) constructed to serve as their main headquarters building and to give the floundering survivors a safe place to live.
Once the floodwaters settled, the Hogwarts survivors attempted to find land, food, and explanations. Unable to reach Hogwarts or the Ministry, they are forced to journey towards Europe and the higher land elevations. They survived by fishing, drinking cleaned water, using Warming Charms against the winter ocean air, and riding the new ocean currents. They end up reaching land in Norway.
When they reached a city, Harry was able to purchase a Muggle ship with what remained of his Galleons to replace their log. With extensive usage of magic, they turn the ship (Harry named it Tranquility) into an unsinkable vessel. With help from a few of their foreign counterparts, they are able to make their way to one of the few remaining magical enclaves. Once they're accounted for, they learn of the new Wizarding city-ship that the Parliament built. Like many others, they made their way there in hopes of finding any survivors of their families.
When they arrive, they learn of Poseidon detecting massive amounts of magic taking place in Antarctica and that many people (magical and Muggle) are feeling an overwhelming urge to journey there. Vague reports of Antarctica no longer being frozen and mysterious King reach the enclave. They send a team of representatives and assassins there to bargain with the King, like many other countries.
Knowing that their surviving friends and classmates are probably still trapped in Hogwarts underwater, Harry and the others gather supplies and willing helpers from the enclave to rescue them. It took a while, but they do manage to find Hogwarts and rescue the trapped survivors (who were still forced to do their magic lessons). The crew of the Tranquility took on hero status to the survivors as they were transported to Poseidon. Dumbledore decides to personally journey to Antarctica when he learns of the failure. Unknown to Dumbledore, Voldemort stowed away aboard his ship.
The teams of magicals sent to Antarctica fail in their negotiations, they become split by those drawn to King Atlantis' power and those loyal to their homes.
With the immediate crisis of the flooding and storms starting to lessen, the people turn their attentions to saving what they can and adapting to their new world. Harry, along with several of the original Tranquility group, starts a business of deep-sea diving to salvage what they can of the drowned cities for the highest bidder. After several dives, they start becoming well-known in both the Muggle and Magical societies. Pirates and Dark wizards start stalking them in hopes of stealing their cargo and making a fortune of their own.
When they reach Antarctica, Dumbledore is able to battle the King to stalemate. However when Voldemort sneaks in and takes the Horcrux, he and the King merge together due their both being soul fragments. United for their thirst of eternal power, they are able overpower and kill Dumbledore.
Harry Potter (14) – captain of Tranquility, broom-flyer
Hermione Granger (14) – navigator, researcher
Ron Weasley (14) – diver, mechanic, broom-flyer
Draco Malfoy (14) – diver, Transfiguration master, broom-flyer
Fleur Delacour (17) – ambassador, Charms mistress
Viktor Krum (17) – diver, Defense master, broom-flyer
Cedric Diggory (17) – helmsman, ward master
Cho Chang (15) – medic, caretaker, broom-flyer
Padma Patil (14) – cook, caretaker
Parvati Patil (14) – cook, caretaker
Fred & George Weasley (16) – inventor, potions master, broom-flyer