As ever, this “quarters” unspeakable oath is an excellent addition to the world of Mythos gaming. It has a range of articles for different Mythos gaming systems, is well layed out(didn't give me a head ache being my metric for this), art is of good quality and seems to have little in the way of errors on a first read.
The content is A grade however, all the standard sections are present and correct from the reviews section though to directives from A-Cell. The other articles seem pretty strong, though I have yet to get my teeth into the scenarios in depth.
The real stand out is Engines Underground, a mysterious manuscript article describing an odd book, which seems to have differing sub-plots for every reader. It is my favourite additions to the Hastur Mythos in some years, and was for me, worth the whole purchase price of Oath 21, on its own.
[4 of 5 Stars!]