Rangers of Shadow Deep: Tenebrous Citadel

From the introduction:

Welcome to Tenebrous Citadel, the final part of a trilogy of supplements that form an epic mission called The Rescue. If you have somehow picked up this book without playing Part 1: Across the Wastes and Part 2: Dungeons Dark, you might want to temporarily put this book aside and pick those up. That said, it is not necessary to have played through those supplements to use this one. In that case, just read ‘The Story So Far’ section, and it will bring you up to speed with everything you need to know. If you have played the previous parts, then you can skip that section as your ranger will have already lived through it!