Helsknight is one of those pirates who is either dumb enough or cocky enough to try and catch a siren. Man has been doing it for years. The only reason he hasn't run his ship into sharp deadly rocks is because Impulse, the siren he is chasing, does not like the feel of sharp rocks on his mer tail.
In all these years, with all 3 past ships lost, with all the nets he tossed into the sea, you would think Hels would just leave Impulse alone before his mad pursuit kills him in one way or another.
Instead Hels finds a barbed rope and as he runs his 4th ship onto some rocks, he jumps out and swings the barbed rope before throwing it and watching it sink into Impulse's tail. The barbs dug in, and as Impy struggled to pull them out, Hels ran closer ready to wrap more barbed rope around the siren to keep him in his grasp. Impulse just got the final barb out of his tail when Hels tackled him and started binding him with the barbed rope before dragging him to the ship.
Everyone on the ship celebrated that night(Helsknight would have killed them if they didn't), and Hels kept Impulse by him for the entire party. As Impulse lay up at night in the captain's bed as the captain forcibly cuddled him, the little siren felt like this was going to lead to the pirate's death.
Despite Hels' obsession, capturing a siren is a great achievement. For many pirates on the ship, this is a goal they could never achieve. For the rare, more sensible ones, they can see the danger it puts them in. Several crew members vanish during the night.
Impulse is exhausted, but he refuses to let himself be used like this. As if he's just a toy, a prize to paraded around. He detests it. He's never liked this game of tag - no matter how hard he tried, he could never get the pirate off his tail.
Clearly he needs to teach Hels a lesson he won't forget. One that means he won't be chased anymore.
He has sharp claws and sharper teeth. He's not going to be a victim.