The Infernal Books
Purpose
The purpose of the Infernal Books is to grant promised power to followers. There are seven total that are basically volumes of the same series, in order to use Book III one has to have mastered Book II and so on. Gifts such as wings, power over fire, the ability to shape fire into solid objects like swords, horns and physical transformations to improve strength and other physical properties as well as bringing new necessities (ex. Book III reforms the Disciple's teeth into fangs, making them completely carnivorous, meaning they cannot properly digest plants)
Acquisition
In order to acquire an Internal Book, one must first commune with the demon lord Cinder, Lord of Fire and Vengeance, and generally he isn't summoned but rather appears to the downtrodden and angry. Those who have lost everything and seek vengeance are very likely to be approached by this lord of fire. Then one must take their vengeance in a very specific way, by sacrificing the one who wronged them, or in some cases someone said person cares about. If sacrificed to Cinder in a certain way, he will take the corpse, and from it's bones, skin and blood create an internal book, which will then give the new disciple basic infernal powers. Other, similar sacrifices must be made to get the other six.
Use
The use of an internal book is similar to the practice of study. One simply needs to read through and fully understand the book to know how to cast this magic, their power coming from Cinder making them akin to Warlocks.
Caveats
To be a disciple is to slowly become a demon. The minute one makes the first sacrifice is the minute Cinder owns their soul, and as they become more demonic, while they get stronger they also inherit the weaknesses of demons, an intolerance for holy things and magic, a weakness to silver and a general weakness for water (which weakens their fire-based magic) and other, similar weaknesses slowly grow as one becomes more and more demonic.