The second supplement to Original Dungeons and Dragons comes from Gygax’s TSR partner and D&D co-creator Dave Arneson. If supplement one was only an addition to the original rules, adding more monsters, items, traps and classes, this is both that and a little bit more. 

Much as in the Greyhawk supplement, the Blackmoor supplement isn’t a source book for the Blackmoor campaign. Much as in the previous supplement, new classes (Monk and Assassin) are added to the game, as well as new monsters with a particular focus on Giant animals (Giant Rats, Spiders, Otters, etc.) and submarine creatures like Mermen and Sahuagin. In fact there’s a whole section with rules for underwater combat. There is also a curious section with rules for dismemberment and “head shots” which is kind of reminiscent of Fallout mechanics.

Where this supplement brings something new is in the first example of a setting and dungeon setup with a little plot to go with it. It’s not fully developed, but The Temple of the Frog, with its location maps, descriptions of history and creatures of the place and detailed description of the temple’s rooms and dungeon as well as the surrounding city, and with the description of NPC relationships and temple social structure does feel like a small campaign setting. It says nothing about Blackmoor castle though, despite the title. It’s all about a weird swamp temple with a Frog god, killer frogs and an alien high priest who has a spaceship and everything. At least it’s imaginative and fun! Read it mainly for the first setting in the history of D&D, which makes it a more entertaining read than the comparatively dry manuals and supplements that came before.