In Glimmering Crypt we are presented with the deepearth, Planet X Games version of the Underdarkn. While there is mention of dark elves, mushroom men, and duergar, this is a realm of blind albino cultists and cyclopean horrors. This dark world is less Gygax and Salvatore and more City of Lost Children, Dark City, and Pan's Labyrinth. Levi gives us an overview of the realms (with rules and suggestions). We're also presented with a cool d20 random encounter table.
This takes us to the Crypt itself. The "Ioun King" is a legendary dwarven wizard and the adventure takes place in his unwitting tomb. The party is presented with rumors to seek this place out and given 21 unique encounters/keyed locations for the heroes to traverse. The adventure also presents 18 stats blocks for new monsters and big bads and 5 magic items (technically 8 because of the Ioun Stone variations).
I think Glimmering Crypt is the most accessible Planet X adventure to the general gaming public. While it is full of references and has some genuinely creepy content, it's not mutant hillbilly, jungle cannibal, or kaiju cultist weird. This isn't a bad thing and I enjoy the adventure. What really sets it apart is the villain. Its quite unique and definitely isn't going to be what the player's expect.
The fun adventure is book ended with faux comic book ads. My absolute favorite of these is the Howling Crater Mutant Hillbilly Teeth.