The Jeju Island Gate was an S-Rank gate that opened on Jeju Island leading to the island being uninhabitable. The creatures exiting from the gate consisted of a variety of powerful ants. The boss of the dungeon that came out of the gate was the Ant Queen.
Ever since the Dungeon Break of Jeju Island had occurred, South Korea had become the only country in the world to lose any of its lands to magic creatures. While many countries openly expressed sympathy, inwardly, they laughed at the apparent incompetence of Korean hunters. When all three reclamation operations failed, the criticism reached its peak.
Due to the island being uninhabitable for an entire 4 years, the ants were allowed to evolve at a terrifying degree. An ant with wings was discovered on the coastline of Japan sparking fear within the community with the Ants evolving to the point where they are able to leave the island.
As a result of this discovery, Japan and Korea combined their forces in an attempt to rid Jeju Island of the ants that inhabited it before the ants left the island to nearby land and wreaked havoc.
The 4th Jeju Island Reclamation Raid was conducted with the Draw Sword Guild of Japan and the S-Rank Hunters of South Korea who were members of the raid team. Japan's goal was to draw the attention of the ants on the island while the Korean S-Rank hunters slew the Ant Queen.
During the raid on Jeju Island, both parties, Japan and Korean hunters were successfully completing their tasks, with little interruption. However, the appearance of a mutated ant named the Ant King led to the annihilation of the entirety of the Draw Sword Guild's S-Rank hunters except for Reiji Sugimoto who did not directly participate. The Ant King, birthed from the Queen as the ultimate soldier, was an immensely powerful magic beast that killed Goto Ryuji and his guild members. The King did not enter Jeju Island from the S-Rank Gate as it was born from the Queen. It was also responsible for the death of Min Byung-Gu as well as for injuring multiple Korean Hunters. The island was ultimately cleared when Sung Jin-Woo appeared, who annihilated the ants on the island including the Ant King ultimately saving Jeju Island and ending the havoc caused by the S-Rank Gate.