Orm desired war with surface to keep Atlantis safe from decades of pollution and environmental damage to the oceans. However, Atlantean law says that in order for a king to acquire the title Ocean Master, he needed support of four of the seven kingdoms (two of which no longer exist). Orm had acquired the support of Atlantis, Xebel and the Fishermen. Orm needed one more so he planned to attack the Brine and force the king to support him so that the war against the surface can be waged.
BattleThe Brine. The plan was to attack straight Just as it looked like Orm of Atlantis was going to slay the Brine King, Arthur arrived with the Karathen, creatures of the sea and the army of the Trench. While the creatures of the sea and the Trench attack small ships and cavalry, the Karathen made short work of major Atlantian, Xebel and Fisherman battleships. This causes the Atlantians, the Xebel and the Fisherman to fall back.
The King Arrives
The Karathen's emergence alerted Orm, who ordered his combined armies to attack the ancient beast. However, the small ships' volley of laser fire didn't affect the beast's thick hide much. The monster continued to wreak havoc among the Atlanteans' heavier more powerful ships, and it rose from the underwater magma of the Brine Kingdom and swam upwards to wreak more havoc to the Atlantean fleet. The Atlantean gunships and Men-At-War opened fire with lasers. But the tide began to turn when Arthur embraced the power of the Trident; it channeled his ability to communicate with marine life. The sharks the Men-At-War rode began to force the Atlantean soldiers off their backs and devour them. Next, a massive wave of sea creatures emerged and swam in front of the Karathen to fight back against Atlantis, ripping their armies and ships apart with the Karathen.
Duel with Orm
With Mera's advice of getting Orm from his element (the seas) into his own (a surficial area), Arthur lured Orm into the water surface and dueled him on a warship floating on the water surface, thus forcing him to fight on land, what he is rather unskilled at. This allowed Arthur to defeat him with a relative ease. Orm than insisted that Arthur would keep the Atlantean tradition of a duel to the death, but Arthur refused and spared Orm's life. When Orm continued to insist, Atlanna showed up and calmed Orm down. Realizing that even their mother is siding with Arthur, Orm full surrendered and Arthur was crowned as the king of the United Atlantis.
After the battle, several of the destroyed Atlantan warships (presumably destroyed by Karathen) rise to the surface as Stephen Shin points out while, on TV, giving further evidence that Atlantis exists. Also, the bursting of Karathen through the seafloor in the Kingdom of the Brine was such intense, that it was measured by human devices.