The Grimorum Arcanorum was an ancient book of magic, dating back to the days of the Roman Empire. It was written by the original Magus, court sorceror of Caesar Augustus. Over the years, it passed through numerous owners, each of whom added something to the book.
The Grimorum eventually came into the possession of the Archmage at Castle Wyvern in the 10th century. The Archmage kept it until 984, when Goliath, Demona and Hudson retrieved it so their Magus could cure Prince Malcolm. The Archmage had used a spell from the Grimorum to enhance a poison he used on the Prince. The book stayed with the Magus, until he was forced to leave it with Mary and Finella when they stayed behind from Avalon. Some time after, the book came into the hands of David Xanatos (sold to him by Mary and Finella), who learned of the gargoyles at Wyvern through the Magus' account of his cursing them in the tome. The Grimorum remained with Xanatos (aside from briefly being stolen by Brooklyn for Demona, who kept a few pages of it) for the next few years. When the Manhattan Clan left the Eyrie Building, they took the Grimorum with them.
They kept it at the clock tower until the Weird Sisters directed Demona and Macbeth to steal it. Once in the Sisters' hands, it was given to the time-travelling Archmage, who consumed it, gaining its knowledge and power. However, when the Archmage lost the Eye of Odin, he was unable to control the Grimorum's power, and both were destroyed.
The Grimorum held numerous spells, including one to make poisons more deadly; one that entrapped the survivors of the Wyvern Clan in stone sleep until the castle rose above the clouds; one that could turn a target into the spell-holder's slave; one that could turn those who saw it cast to stone by night, until the sky burned; and many more. It also contained historical accounts by some of the owners, such as that of Castle Wyvern's Magus.