The Jade Passage is the first of two dungeons that make up the final quest of the game. You will find some nice items here, as well as nasty enemies. Fortunately, you can Teleport out whenever you want, and return trips will be quite a bit more efficient when you don't need to gather treasure.
For a final dungeon, the Jade Passage certainly has a unique look. One of the most colorful dungeons in the entire game, it's an almost tropical cavern. Don't let the greenery fool you into thinking that it isn't dangerous, though.