Lord Orm

Lord Orm was the villainous and sadistic brother of Aquaman, who stole the throne from him during a time of tension between Atlantis and the Surface World. He is based off of the DC Comics villain Ocean Master but is ultimately more evil than his mainstream counterpart.

History
Orm first appears as Aquaman's brother and a seemingly loyal supporter of his sibling's rule. However it later turns out that Orm attempted to have his brother assassinated and made to look like the surface world had done the deed. In doing this, Orm hoped to become the new king of Atlantis and to also start a genocidal campaign against the surface world, which he viewed as inferior.

Though Aquaman survived the attempt on his life, upon returning to Atlantis he found that the damage had been done. Orm had seized power and with his loyal minions had Aquaman imprisoned. Soon capturing the Justice League as well, Orm used their arrival as an excuse to declare war on the surface world. Using a radiation-powered doomsday weapon he intended to melt the polar ice caps and with that flood entire cities.

To ensure that his rule remained absolute and unchallenged, Orm also left Aquaman and his infant son to die, not counting on Aquaman's determination to free himself and save his son from death.

With the Justice League at his side, Aquaman took the fight to Orm and personally engaged his sociopathic brother. As Green Lantern and Batman disarmed the doomsday weapon, the ground gave way beneath Orm's feet during his fight with his brother. Clinging on to the edge for dear life, Orm pleaded for his brother to save him, but an unmoved Aquaman instead grabbed his discarded trident and let Orm fall to his apparent demise.

Trivia

 * Though seemingly final, the show producers noted that Orm's death was sufficiently ambiguous that they knew they could always bring him back if they wanted to, but never did. This suggests he likely died.
 * He was voiced by Richard Green.