From the PDF introduction
A Web Enhancement for the Book of Challenges
The Book of Challenges offers dozens of exciting traps, mazes, and other intriguing dungeon features to keep the player characters (PCs) on their toes. This web enhancement by the book’s editor offers Dungeon Masters (DMs) an idea for what to do if the characters try to circumvent the dungeon’s challenges with a quick ethereal jaunt! To use this web enhancement, you must already have the Book of Challenges. This bonus material is exclusive to the official DUNGEONS & DRAGONS website.
Dungeon Masters take care and setup intricate encounters focusing on dungeons, mazes, and surprises. One challenge they may face is that a party of characters utilizes spells and magic items to circumvent their elaborate plots easily. For instance, a character with ethereal jaunt can scout ahead, find all the correct turns in an intricate maze, and locate all the opponents, allowing the rest of the party to bypass the danger.
Sometimes player characters need a reminder that the Ethereal Plane is not their personal playground.