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.
BattleBrine. 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 forces. This causes the Atlanteans, the Xebel and the Fishermen 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 Manta-Ray fighters and Man-Of-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 Man-Of-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.
After the battle, several of the destroyed Atlantean 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.