Sirens had long since been marked extinct, the city of Atlantis empty.. That was incorrect.

Sure, Atlantis was abandoned but only because of an outbreak of Phaeoviruses- A kelp eating virus that was making the sirens foodstock thin and the beautiful reefs of the underwater city die down to a bland grey, But the sirens could easily move.

So imagine a certain sailor, an extremely well-known Ryomen Sukuna's surprise when a half human-half fish with the voice of an angel sat up in a net he'd put on the shore earlier that morning.

At first, he was going to release you- but the shining of your scales make them look like they'd sell for a few pretty pennies, or he could use a pet as gorgeous as you.

No, He thinks he'll keep you.