Unfortunately, it is not possible to fight the Undying King in Adventure Mode in the game. This is because the Undying King is a boss that is specifically tied to the main campaign storyline. In order to face off against the Undying King, you will need to play through the main campaign.
Adventure Mode in the game is designed to provide players with a more randomized and replayable experience. It generates new worlds with different biomes, dungeons, and bosses for players to explore and conquer. However, it does not include all the content from the main campaign, and certain bosses like the Undying King are not available in Adventure Mode.
If you are specifically interested in fighting the Undying King, you will need to reroll the main campaign and progress through the story until you reach the appropriate point. The Undying King is encountered in Yaesha, which is the third world you visit in the main campaign. You will need to complete certain objectives and defeat other bosses before you can face off against him.
In my personal experience with the game, I found the Undying King to be a challenging and thrilling boss fight. The fight requires good coordination, strategy, and awareness of the boss’s attacks and patterns. It’s always satisfying to finally defeat a tough boss like the Undying King and progress further in the game.
To summarize, the Undying King cannot be fought in Adventure Mode. You will need to play through the main campaign and reach the appropriate point in order to face off against him. Keep in mind that the Undying King is a formidable opponent, so be prepared for a challenging battle. Good luck on your journey!