Royal Phantom
Danny had been living with Bruce and his kids for a while now, he's not only made himself part of both sides of their lives, he's also helping Gotham heal by slowly undoing the curses placed upon the city.
He's also found a balance between his "Wayne Life" and his Ghost King duties. But that all changed when he was called on a Justice League related emergency that involved Ghosts near the coast (Vlad up to no good).
It started out as just a regular fight between the two Halfa's. Until Danny was soaked by the Salt Water by kne of Vlad's attacks. He began to go through a change in his physical appearance, gills appearing on his neck, webbing growing between his fingers, his legs forming into a tail that mermaids have, and his face gaining some scales.
The pain he felt in the transformation had caused him to fall into the shallow sea water, that's when all those born of the sea could feel it, even Aquaman, the Siren King's return.
During the Era of the Fentonightgale's, one member of the family had fallen in love with the Siren King of that time. She kept it a secret from their family, fearing that they would be separated, or worse. And when she had the baby of the Siren King, she told her family that the father had perished at the hands of a Witch, knowing that they would believe it. She hated how she had to leave her love behind when the family moved on, she couldn't even sneak away to say goodbye.
The genes of the Siren Kimg had lay dormant within the Fentons for many years, it didn't pass on to Jazz, with her not having the Dark Hair of the Siren King's bloodline. And now that Danny has awoken the genes of the Siren King in him, the Siren Kingdom has resurfaced once again, they have long been in hiding, fearing that they would be wiped out without the strength of the Siren King, and now they will do anything to get Danny for their conquest.