The Creature Crucible series was a set of four splatbooks released for the BECMI ruleset of Dungeons & Dragons. Similar in nature to The Complete Book of Humanoids, although actually predating that book, each of these splatbooks provides mechanics and setting information relating to an array of new monstrous PC options, a departure from the Human/Elf/Dwarf/Halfling core of BECMI. Alongside the new races, each also examined a new region of Mystara particularly associated with those races; in this, they were something of a side-branch of the Known World Gazetteers.
PC1: Tall Tales of the Wee Folk
It also includes new spells taught amongst the more magical breeds of fey, some new fey-related equipment, new skills, an expose on the Dreamlands (a magical, primeval region of forest that is effectively the capital of the fey's presence on Mystara), and an assortment of adventures aimed at fey PCs.
PC2: Top Ballista
This splatbook focuses on the Flying City of Serraine, a magical city crafted by Mystaran Gnomes and turned into a roaming beacon of magical learning and development. This was the book that brought the gnome race from Advanced Dungeons & Dragons to BECMI, with the following collection of new races:
It also includes new mechanics for crafting mad science gizmos (ironically less obnoxious than those given to the Tinker Gnomes of Dragonlance), new magical items common in Serraine, new rules for flying the magitek biplanes that protect Serraine from invaders... yes, Mystaran skygnomes are competent, and the obligatory new skills and new adventures to take advantage of what its added.
PC3: The Sea Peoples
As is name implies, this book focuses heavily on adventuring underwater, and has the largest array of new rules and rule modifications of any book in the series. It also has the smallest number of new PC races, consisting of:
The obligatory new skills, new gear and undersea adventures are, of course, included, as is an expose covering the vast underwater areas of Mystara inhabited by these creatures.
PC4: Night Howlers
The last of the Creature Crucibles, Night Howlers is also the most unusual; it's a guide to the Therianthropes of Mystara, including their history on that particular world and full rules for playing shapechangers!
This book also includes two new monsters (Lupins and Chevalls), an assortment of new therianthrope-related magical items, a mini-gazetteer on "The Valley of Wolves" - a werewolf nation, and details on the small group of Mystaran Immortals who have portfolios connected to werebeasts; Kaladan, Koryis, Mrikitat, Ruaidhri Hawkbane, Leo Variantia, and Zirchev.