An obscure reference that first appeared in the D.C. comics in 1966, Necrus is the name of an underwater city. However, unlike Atlantis, it is quite unstable as it only appears in the physical realm for brief periods of time. And even when it is visible, it has no fixed location. Also known as the "Black City," its militaristic society is ruled by a tyrant named Mongo, who once went to war with Atlantis for its acceptance of surface dwellers.
And while nothing is confirmed, this does line up with the plot of the previous movie, which saw Arthur Curry's half-brother, King Orm (