If the Samurai island is crowded, consider switching servers to find more spawns.
The most reliable (though tedious) way to get the Mysterious Key is by defeating specific enemies. However, they do not drop from standard early-game mobs like Slimes or Goblins. You need to target high-level or rare-spawn enemies. how to get mysterious key in critical legends
Once you have the key, head to the in the center of the Whispering Woods to claim your reward. If the Samurai island is crowded, consider switching