Visiting the grave of Aquaman’s fire, Manta destroys it asking the rhetorical “Where is your son?” before Mera’s people show up to attack him….and get their asses kicked by him. Before the fight can get too far along, however, a voice stops them, and calls Black Manta “David”.
It’s Siren telling him that they need to work together to find his son, as she makes a hard water image of the new Aqualad.