This product is absolutely necessary for running the Gracklstugh chapter in Out of the Abyss, regardless as to when your party visits it. It deciphers the chaos that is perhaps the least well organized section in any 5e module to date, provides content to further invest characters, help scaling the large dungeon beneath the city, and both clarifies and expands upon aspects of Gracklstugh society and the factions therein that have proved incredibly useful during my time running the chapter.
I cannot recommend this product enough. If you intent to run Out of the Abyss, or even if you just want to use the city of Gracklstugh itself for some other purpose, I believe you should really consider picking up Turmoil in the City of Blades.
[5 of 5 Stars!]