After cameoing in Batman v Superman: Dawn of Justice and making his debut later this year in Justice League, Jason Momoa's Aquaman will strike out on his own late next year. Aquaman plot details are still being kept under wraps, but the casting announcements have made it clear the water-based superhero will have a lot of nefarious figures to deal with in his first DC Extended Universe solo adventure. For instance, Aquaman will have to deal with the machinations of his half-brother Orm, a.k.a. Ocean Master. If that wasn't bad enough, Orm could also be the one who's ruling over Atlantis during these events.
Jason Momoa posted the above photo on Instagram of him hanging out with Patrick Wilson, and he first referred to himself and his costar by their character names: Arthur (Aquaman's civilian identity, Arthur Curry) and King Orm. Like in the comics from recent years, both Arthur and Orm are both royalty, as they share the same mother, Queen Atlanna, But it's generally been assumed that Aquaman would be the one sitting on the Atlantean throne. That could still happen at some point during the movie, but assuming Momoa isn't screwing around with the fans, when we meet Orm in Aquaman, he'll be the one presiding over Atlantis rather than his half-human sibling.
Fortunately, having Ocean Master be the king of Atlantis isn't unprecedented. In the New 52, Orm ascended to the throne after his father is killed and his mother mysteriously died. Years later, Arthur Curry found Atlantis for the first time (he'd been raised on the surface by his human father, Thomas Curry), but despite being older than Orm and thus entitled to become the new king of Atlantis, he let Orm stay king. It was only after the Throne of Atlantis storyline, where Orm tried to invade the surface, that Aquaman overthrew his brother and accepted the responsibility of ruling Atlantis. Perhaps a similar conflict will occur in the Aquaman movie, and towards the end of the movie, Arthur realizes that for the greater good, he has to take the throne so his brother can't hurt others with his royal power.
Along with handling whatever trouble Ocean Master is cooking up, Aquaman will also face off against his most famous adversary, Black Manta. Then there's King Nereus, who is "not quite villain" according to director James Wan, but will reportedly want Aquaman dead so he can claim Mera as his own. If Arthur Curry figured he was going to get a respite after fighting Steppenwolf and his Parademon army with Earth's other superheroes in Justice League, he thought wrong.
We'll find out what the deal with Ocean Master is when Aquaman dives into theaters on December 21, 2018.