Lord Orm was the villainous and sadistic younger brother of Aquaman.
Orm hired Deadshot to murder his brother, and then took over the throne as regent of Atlantis. He planned to lead a war against the surface-dwellers. Orm used his nephew as bargaining chip to ensure Mera's cooperation, and then took him to a cliff near a volcano, where he strapped him next to Aquaman. Orm gloated about killing them both, blaming it on the surface-dwellers, and thus ensuring his rightful inheritance to the throne. He departed, leaving Aquaman and his infant baby to plummet to their deaths.
Afterwards, he used the plutonium from a U.S. nuclear submarine to arm the Doomsday Thermal Reactor, an Atlantean weapon of last resort designed to melt the polar cap, thus submerging the entire planet. When Aquaman - having cut off his own hand to save his son - tried to disarm it, Orm confronted him, and had the upper hand thanks to the Trident. However, amidst their fight, the ice bridge collapsed beneath Orm, who grabbed on to its edge.
He begged his brother for help, but Aquaman refused, accepting his brother's fate, and Orm fell down the abyss to his demise, paying for his actions.
In the DC Comics, Orm is best known as Ocean Master, one of Aquaman's archenemies. He has tried at various times to kill his half-brother and claim the throne of Atlantis for himself. Bruce Timm has stated the reason Orm never went by the name "Ocean Master" in the series, was due to opinion on the name sounding too silly for use in the show.
According to the commentary for "The Enemy Below", Timm mentioned that the creative team originally considered bringing back Orm but now in costume like in the original comics. The idea was scrapped because it would require an explanation about how he would've escaped from the cliff.