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The corebook provides everything you need to play a game of Joan of Arc: the Roleplaying Game.
First third of the book concerns characters and character development, including Origins, Backgrounds, Classes and Feats. Compared to other 5e products, Joan of Arc details only the human race, but provide many different Origins, each one being a broad region of Europe your character might come from. Backgrounds cover the typical social classes that you could find in Medieval Europe. Finally, there are 4 classes available, each with their own specializations and aptitudes. One particularity of the game is all characters are Christians, and part of the system translates Christian belief into the game (for example cardinal virtues or sins)
Second third of the book cover the game rules : tests, saves, combat and mass combat. One particular aspect of the game is the use of the characters' beliefs as a resource during the game. Faith, Superstition and Pragmatism are the 3 core beliefs of all characters, and the sum of all 3 is always the same. During the game, those points fuel aptitudes or modify some tests. Some opponents can also use those to target some characters specifically (for example, a character with a high score in Superstition will be more sensible to a witch' powers)
Last third of the book is GM's domain, with a detailed presentation of the setting, major NPCs, creatures and opponents, as well as a short scenario. Even if the game provides information for most of Europe, the default setting is the Hundred Years War, between France, England and Burgundy. More details are provided about that one, but some information is also provided about other possible settings (including trade wars in the Baltic, Family feuds in Italy or the defense of Christendom against the Ottoman Empire)