A grimoire is a "textbook of magic," and is wielded by a necromancer. It can perform spells and summon Duras or other supernatural beings.
The three most powerful grimoires are, The Key of Raziel (Held by Reiga), The Book of Solomon, (Held by Takashirou) and Enoch's Key, which the two groups are both searching for.
Enoch's Key Edit
Up until 1000 years ago, the former supreme leader “Ariou” or "Giou-Sensei," possessed this grimoire. But after the “Sunset of the Underworld,” the location of “Enoch’s Key” was lost until it was found again by Takashirou, who sealed it in a shrine at a Demon Gate.