Patrick Wilson will be playing the villain, Ocean Master, who is the half brother to Momoa's Aquaman.
James Wan confirmed the news on Twitter.
"In casting news that no one saw coming from 20,000 nautical miles away, I ordain @patrickwilson73 the Ocean Master. #KingOrm #Aquaman," Wan posted
In the comics, Ocean Master, aka Orm, grew up jealous of Aquaman's powers while he he had none. Eventually Orm used tech to give himself power and became the villain Ocean Master.
Aquaman has an October 5, 2018 release directed by James Wan starring Jason Momoa as Aquaman, Amber Heard as Mera, and Willem Dafoe as Nuidis Vulko.