Though no formal name has ever been attributed to them, these were a race of highly evolved, sentient jellyfish. They despised the notion that the seas were dominated by the humanoid Atlanteans, and embarked upon an ambitious plan to insure racial purity in their territories. They bided their time until Atlantis was at its most vulnerable, when its capital city Poseidonis was under the command of an inept leader named Pletus. The Jellyfish commandeered ships and transport craft and began their invasion. They always concealed themselves within metal shells, so that nobody ever knew what the invaders actually looked like.
In times of war, it was customary to bequeath complete authority in Atlantis to the highest ranking military commander. Although the general fought valiantly, he was killed in the first attack, thereby solidifying Pletus' bid for power. Taking control of the military, his poor judgment misdirected battles and caused a severe drop in troop morale. Ultimately, he surrendered Atlantis to the jellyfish and was summarily drawn and quartered. The Atlantean ruling council were then apprehended and torn to shreds, their remains left floating in the Atlantean waters. The jellyfish rounded up as many dignitaries as they could find and incarcerated them at the Aquarium as political prisoners. To strengthen their own army, the jellyfish found nomadic tribes from the outlying settlements around the continent and pressed them into service. Although these men had no desire to wage war against their fellow Atlanteans, they feared that the jellyfish would execute their families if they tried to resist.
At the height of their occupation, Atlantis' former king, Aquaman, returned to his home after having been away for several years. He was captured by the rebel army and placed inside Aquarium. Aquaman conspired with the other prisoners to lead a rebellion against the invaders. He knew of secret aqueducts that could be used to leave Aquarium and get back to the royal palace.
They soon discovered that the invaders were rounding up random Atlantean families and taking them to a processing plant where they were being executed. Aquaman soon determined the true nature of the invaders and realized that they were in fact sentient jellyfish. He rounded up as many resistance members as he could and had them mass produce canisters containing a poisoned specially designed to kill jellyfish. They placed them at strategic locations around the royal palace and detonated them. Though the poison was deadly to jellyfish, it was harmless to other sea life. All of the invaders inside the capital city were killed instantly. Some managed to escape and retreated back behind the dome perimeter. The invaders were not finished however. Although they no longer controlled Poseidonis, they were able to erect a blockade around the domed city, cutting Atlantis off from their supply chains. If they couldn’t kill the Atlanteans directly, then they would starve them out.
An Atlantean general led the charge to push the invaders back, but the opposing force was too strong. They simply didn’t have the military strength to press forward. Aquaman ascended the highest point in the city and broadcast a telepathic command to every fish, cephalopod and marine mammal that he could summon. Taxing himself beyond measure, he succeeded in raising an army that obliterated the opposing forces once and for all. Atlantis was liberated. Three of the giant jellyfish managed to escape the final assault and flee. They have never been seen again and their final fate is unknown.