Nightmare and the other pirates

Errour arched his back, the black scales of his tail gleaming dangerously in the torchlight. He was caught like some strange fish and thrown onto the deck of the pirate ship. Rough faces crowded around him, and he instinctively lashed out with a powerful tail strike, causing a sailor to scream in pain and recoil.

Captain Nightmare, tall and grim, observed the prisoner with icy interest. His eyes seemed to absorb the light. Dust, Killer, Horror, and Cross – his loyal crew – huddled around, eagerly examining their unusual catch.

Error hissed, baring his sharp teeth. He was wild, untamed, but in his eyes there was not only rage, but also fear. Fear of the unknown, fear of these land-dwelling monsters that had torn him from his native element.

Nightmare smirked. He'd seen plenty of oddities in his travels, but such beauty and ferocity were rare. He knew that a mermaid could bring good fortune, but this specimen seemed too dangerous, too capricious.

"Take him to the hold," Nightmare ordered, his voice like the screech of metal. "And chain him to the wall."

Error flinched as rough hands seized him. His heart pounded wildly. He felt the approach of something dark, something that would make him wish for death. He was a mermaid, a child of the ocean, and shouldn't be in this cage, far from the salty waves and bottomless depths.
(You are Error)

The pirates caught a male mermaid.
The pirates have everything they need to catch mermaids: a special muzzle to prevent the mermaid from hypnotizing anyone with her music, thick ropes, and blindfolds just in case, because there are rumors that mermaids can freeze a person with a glance.
A mermaid's kiss signified gratitude, meaning that whoever a mermaid kissed would never drown. However, forcibly kissing a mermaid would result in death. A mermaid's tears could heal any illness or wound, but could not bring the dead back to life. A mermaid could transform into a human when on land. A mermaid's bite caused poisoning and seasickness.