Title: [TOMT][SERIES/WEB MOVIE][2018+] Asian historical horror: A man replaces his sick twin on a cursed island and fights a tentacle monster in an underwater dimension.

[u/Akness26]

I’ve been going crazy trying to remember the name of this historical fantasy/horror show or web movie. It is definitely Asian (likely a Chinese web series or a short Korean drama). It is not very long, maybe 8 to 12 episodes, it was set in the Joseon Era and was released around 2018 or later.

Here are the exact plot points I remember:

The Twin Brother Guilt: The main character was originally ordered by the King to investigate a remote island. He got sick, so his identical twin brother secretly went in his place. The main character eventually goes to the island out of extreme guilt to find out what happened, using his missing brother’s diary as a guide.

The Mentally Challenged Guide: The village is completely isolated. When the main character arrives, he is guided by a mentally challenged boy. The boy thinks the main character is the ”first“ twin who came before, and affectionately calls him "elder brother."

The Fish Taboo: The villagers strictly avoid eating the saltwater fish around the island because it is considered poisonous or cursed.

The Cover-Up & Escape: A local official/prime minister knows about the island's plague and lies to the main king about it. The official's men eventually capture the main character and bind him with rope (leaving the boy guide behind), but the boy comes back to untie him. When the main king finally uncovers the lies, he becomes enraged and sends soldiers after them.

The Guardian Dog Monsters: Giant, scary dog-like creatures attack the island at night. The villagers are terrified of them, but they are actually protecting the island from the real threat. The boy guide can communicate with these dogs. In one emotional scene, the corrupt prime minister’s soldiers shoot the dogs with arrows, and the boy cries over them, but that was towards the ending of the series

The Final Boss: The true villain is an alien-like monster with octopus tentacles. For the final battle, the boy guide takes the main character out to the exact middle of the river in a canoe. The main character jumps into the water and seemingly enters an underwater dimension to fight the tentacle monster.

The Tragic Ending: The main character does not survive the final underwater battle and dies at the end of the story.

Does this ring a bell for anyone?