Grimoires are ancient books, full of spells and rituals, used to invoke the dark.
Sometimes they are employed by witches, but most often the user is a mage. The spells contained in a grimoire must be followed to the letter, or death can be the result. One mispronunciation of a magic word, and the consequences can be catastrophic.
Most of these famous texts have been lost (such as the Patrixa and the Key of Solomon). The most dangerous and powerful of all grimoires, however, were written in the Old Tongue by the first men of the county. Primarily used to summon daemons, these books contain terrible dark magic. Most have been deliberately destroyed or hidden from human sight. The most mysterious and reputedly most deadly of these is the Doomdryte.
The Pendle Witch clans have their own grimoires, but they never contain the ritual for summoning the Fiend. They consider this too dangerous to be written down: it is learned by heart and passed down through the clans from mother to daughter.