Splintering of Atlantis
Before its descent into the ocean, the seven kingdoms of Atlantis were as one and ruled by King Atlan. After testing done on his trident sank the empire into the sea Atlan, ashamed of what he did, hid himself away with his trident. Sometime after the fall, the seven kingdoms split up into their own dominions. Of the seven, two - the Deserters and Lost Kingdom - did not survive, while the remaining five evolved into the different cultures, governments, and lifeforms.
The legend of King Atlan and his trident survived amongst Atlantean people throughout the years, proclaiming that only the ‘true king’ of Atlantis could wield it and reunite the seven kingdoms.
Queen Atlanna's Decision
The royal lineage of Atlantis ran from eldest born to eldest born for thousands of years. In the 1980s, the then Queen of Atlantis - Atlanna - ran away from an arranged marriage from Orvax Marius, washing up on the shore near a lighthouse in Maine. After being rescued by and falling in love with its keeper, Thomas Curry, the couple produced a son named Arthur. After an attack on their house, however, Atlanna returned to the sea to marry Orvax, leaving her eldest son behind with his father.
After her return, Atlanna married Orvax and produced a son, Orm Marius. She also made her vizier Vulko promise to protect Arthur and train him in the Atlantean ways. At one point, however, her husband discovered the existence of Arthur, and for this, executed the Queen by exiling her to the Trench.
By 2018, her younger son, Orm, was on the throne of Atlantis and planning to launch an attack on the surface world for its crimes against the sea. Working to prevent a needless war that would cost thousands of lives, two Atlanteans close to Queen Atlanna - Vulko and Princess Mera - began to formulate a plan to bring Atlanna’s first born, and technical rightful heir, to the throne of Atlantis by having him retrieve Atlan’s legendary trident.
In early 2018, around one year after Steppenwolf's attempted invasion of Earth, Arthur Curry was known around the world as the oceanic superhero Aquaman. Deep in the seas, Jesse Kane and his pirates, one of which was his son David, led an attack on a Russian Naval submarine, successfully managing to kill several sailors including the captain. Not long after the attack began, Arthur arrived and engaged in a duel with David Kane, which led to Jesse Kane firing on Arthur. However, this plan failed, and Arthur merely deflected the weapon and wounded Jesse Kane's shoulder instead. Jesse shot at Arthur in retaliation, but missed and instead burst open several water pipes and knocked a warhead onto his legs. Despite David asking for his help to free his father, Arthur left the flooding room without saving Jesse. Wanting his son to leave before it was too late, Jesse activated a hand grenade, killing himself as his son escaped the sinking submarine on his own underwater vehicle.
In order to wage a war against the surface world, Orm first needed the support of four of the seven kingdoms of Atlantis, as laid out by Atlantean law. At the same time, uniting four of the kingdoms in this manner would grant him the title 'Ocean Master', as well as control of the greatest military might on the planet. As a first step in this plan, he invited King Nereus of Xebel to the Council of Kings, a pre-fall Atlantean meeting place of the heads of the seven Kingdoms, to invite him to join his war. During this meeting, however, a submarine believed to be controlled by surface dwellers began to shoot missiles at the congregation. As the ancient meeting place crumbled, Orm took action and disabled the submarine, the attack of which convinced Nereus to support Orm in a war against the surface.
Later, it was revealed that the submarine that fired on the council was the very same hijacked by Jesse and David Kane, who had been paid by Orm to forge an attack on the Council to convince Nereus to join his cause.
Following the surface dweller’s supposed attack on the Council of Kings, Orm retaliated by sending massive tsunamis to hit coasts around the world.
Arthur was driving home on a coastal road with his father, Thomas Curry, at the time of the attack. The two got caught up in the wall of water hitting the coast, and Arthur was separated from his father, still stuck in the car. Using her hydrokinetic powers, Mera was able to clear the water away from the Curry’s overturned car, calm the wave, and extract the water from Thomas’ lungs, saving his life. In all, the first strike caused millions of dollars in damage, pushed years of trash back up onto beaches, and beached at least one U.S. Navy ship. It also provided further proof to Atlantis-believer, Dr. Stephen Shin, that Atlantis both existed was angry at how humans had treated the oceans.
Shortly before the attack, Mera approached Arthur and asked him to come to Atlantis to take his rightful place on the throne, but he refused. After seeing the aftermath of the attack on his home and the rest of the world, Arthur was convinced to go to Atlantis to stop Orm’s war against the surface.
After agreeing to help stop Orm, Mera brought Arthur to Atlantis, where they met Vulko in an old shipwreck on the sea floor. While discussing the legends behind and whereabouts of King Atlan’s Trident, the three were ambushed by Atlantean soldiers looking to arrest Arthur for illegal entry into Atlantis.
After his arrest, Arthur was brought before his half-brother Orm in the Atlantis throne room, where the two met for the first time. Orm proceeded to explain why he wanted to wage a war against the surface and provoke Arthur, who unknowingly invoked the Combat of the Kings. Despite Vulko and Mera’s attempts to dissuade Orm from accepting, Orm nevertheless did so as an opportunity prove his right to the throne over his older brother's in front of all of Atlantis.
Donning some secondhand armor from an old Atlantean armory, Arthur met Orm in the Ring of Fire - an ancient amphitheater ringed by lava - for their duel. After a a decent fight from both, Orm managed to launch an aggressive attack on Arthur that split their mother’s trident in two. Before he was able to deliver his final blow, however, Mera used her hydrokinetic powers to hit Orm with a vortex of water and rescue Arthur in her ship, fleeing with him from the arena and Atlantis.
While in Sicily tracking down the location of King Atlan’s trident, David Kane and the Men of War arrived to eliminate Arthur and Mera per Orm’s orders. After attacking the two in some ancient ruins, Kane called identified himself as Black Manta for the first time. Manta, seeking revenge for the death of his father, went after Arthur, while the Men of War pursued Mera across the rooftops of the town. Equipped with his new top-of-the-line Atlantean weaponry, Manta inflicted far more damage on Arthur than he had done in the submarine, and only stopped after Arthur threw chained bollards at him that effectively imploded his helmet. Meanwhile, after giving defeating several soldiers, Mera finally defeated all of her pursuers by using her hydrokinetic powers on wine to effectively spear them.
In the aftermath of the attack, Kane was presumably left for dead and Mera stole a ship and took the injured Arthur with her towards the location of the trident.
After fleeing Sicily and in pursuit of Atlan’s Trident, Arthur and Mera discovered that their map had led them to the location of the Kingdom of the Trench. Braving increasingly stormy conditions, Trench creatures began to swarm their boat. After lighting an emergency flare, Arthur realized the monsters were afraid of the light, and with Mera dove straight into the mob of Trench surrounding their boat. After successfully navigating through the creatures, however, the two were sucked into a vortex that even the monsters would not go near, and taken to the Hidden Sea.
After successfully coercing the Fisherman Kingdom to join him, Orm turned to convince the Brine Kingdom to support him and his quest to both become the Ocean Master and launch a war against the surface. Due to past hostilities between their two kingdoms, however, Orm and the Brine King met on the battlefield. The Battle of the Brine thus started with Orm commanding the forces of Atlantis, Xebel, and the Fishermen Kingdom against those of the Brine.
Soon after the battle began, however, Arthur Curry, Mera, and Queen Atlanna rose from the Hidden Sea on the back of the Karathen. After proving his worth to the ancient sea creature, Arthur now wielded Atlan’s Trident, which amplified his marine telepathy and could thus command every living sea creature, which ranged from the Karathen, to whales, and to the de-evolved savage Trench. As such, Arthur turned the battle in on itself, scattering Orm’s forces.
Effectively stopping the large-scale battle, Arthur met Orm above the sea and resumed their duel, now wielding Atlan’s Trident as the true heir to the throne of Atlantis, this time with in an environment Orm, not himself, is unfamilier with. In front of all the armies, Arthur defeated his half-brother, becoming the new King of Atlantis and thus stopping Orm’s proposed war on the surface.
As a result of the conflict, Arthur became the new King of Atlantis, and through his connection with Atlan’s trident reunited the separate Kingdoms of Atlantis, as legend predicted. Furthermore, he took on his role as the bridge between Land and Sea and wholeheartedly accepted his role as “Aquaman.” As a consequence of this, Orm also lost his throne and was arrested by the Atlantean guard.