This is an old school core rulebook, scanned to PDF with no bookmarks. It is basically indistinguishable from Call of Cthulhu 5th edition in style and substance. It has the crunchy game system section with the endless weapons/equipment tables and pointless movement rules that wouldn't look out of place in D&D 3rd edition . The skill set is both smaller and more useful than the Call of Cthulhu, which is a big plus for anyone who ever tried to create a character in BRP. It has a rather slim but highly useful setting chapter, which gives you the flavor of the period for those of us that slept through history lessons. Basically it all boils down to this: Dark Ages were called that for a reason – it was full of desperately poor and ignorant people living short and brutish lives under soul crushing oppression. A perfect setting for cosmic horror, I am sure you'll agree.
Next comes the Magic and Bestiary chapters, both are surprisingly good, or at least better than their Call of Cthulhu equivalents. The concept of limbo is introduced in the magic chapter, non-mythos magic is also discussed and of course there is the obligatory spell list, which is mercifully short and to the point. There are some new monsters and deities in the bestiary chapter. They tend to be less on tentacle rape and more eat your soul side. Each has roughly one page description and an illustration.
Last but not least is “The Tomb” scenario. Except for lack of clear reason for investigators to get involved, it is a solid adventure with lots of mystery, exploration and gratuitous violence. Typically for old school, a TPK is likely half-way through the plot and virtually guaranteed at the end, but it is still a fun scenario to play none the less.
Overall it is a passable rulebook for those nostalgic for the good old days, whether it is worth the price for the rest of us, is another matter entirely. Barely grade C from me.