It was originally written and or compiled during the reign of Caesar Augustus, from 27 BC to 14 AD, by his chief advisor, the original Magus. It was the most powerful and comprehensive book of mortal spellcraft in existence. Ancient magical scrolls and other texts were cut apart and bound into the volume. Spells were written primarily in Latin and Gaelic, though other languages such as Greek were likely represented. Most, if not all, of the spells originated in Europe and other western lands. Some spells may even have been contributed by Merlin himself.

The book passed from hand to hand until, by 971, it came into the possession of the Archmage. When the Archmage was banished from Castle Wyvern in 984 for attempting to overthrow Prince Malcolm, he took the Grimorum with him. Goliath, Demona, and Hudson pursued him to his cave and recovered the book after the young Magus required a spell from it to cure Prince Malcolm of the Archmage’s poison.

The Grimorum remained in the Magus’s possession for the next ten years, until Hakon sacked Castle Wyvern. The Viking leader seized the book and, contemptuous of its magic, began tearing out and burning its pages. When the gargoyles came to the aid of Hakon’s human prisoners, the Magus, blaming them for Princess Katharine’s apparent death, used a spell from the Grimorum to place them into stone sleep until the castle rose above the clouds. Upon learning that Katharine was alive and had been rescued by Goliath, the Magus repented, but one of the pages Hakon had burned contained the counterspell, leaving him unable to undo the curse. At Goliath’s request, he instead turned the gargoyle leader to stone with the same spell.

The Magus later took the Grimorum to the court of Kenneth II, using its magic to disguise the Eggs and help Princess Katharine escape after Constantine III’s coup the following year. Using another spell, the Magus transported himself and his followers to Avalon, where their way was barred by the Weird Sisters. He reflected one of their attack spells back upon them, transforming them into owls, but because the Grimorum was powered by human magic rather than Third Race magic, he could not bring it into Avalon. Knowing that the book contained the spell that opened the way to Avalon and fearing it would fall into Constantine’s hands, he entrusted it to Mary and Finella for safekeeping.

Mary and Finella returned with the Grimorum to Scotland, pursued by Constantine and by his hired sorcerer, Brother Valmont, who coveted the book as one of the Three Keys to Power. At the Battle of Rathveramoen, Valmont seized the Grimorum from Finella and used it to summon a rain of fiery arrows that slaughtered combatants on both sides, including the mate of Demona’s Second, Magus the horse, and Maíl Brigti. Brooklyn wrested the Grimorum from Valmont, and Demona used a counterspell to end the magical assault, though those slain by the arrows remained dead. Brooklyn later tricked Demona into returning the Grimorum to him and Finella, after which the Phoenix transported them and Mary to the United States in the 1970s. They later gave the book to Xanatos, who kept it at Castle Wyvern after relocating the castle to the top of the Eyrie Building.

Shortly after the gargoyles’ awakening, Demona deceived Brooklyn into stealing the Grimorum for her so that she could use it to seize control of Goliath, turning him into a mindless servant. When Brooklyn realized what he had done, he reclaimed the Grimorum, though Demona managed to tear out several pages beforehand to keep for later use. She used one of these stolen spells to help create Coldstone, and another to cast her stone-by-night curse over New York City.

When the gargoyles were forced to leave Castle Wyvern for the Clock Tower, they took the Grimorum with them and safeguarded it there. It remained protected until Demona and Macbeth stole it under the influence of the Weird Sisters, who then delivered it to the Archmage.

Unable to bring the book into Avalon directly, the Archmage consumed it, making the Grimorum part of himself and absorbing its power. When Goliath tore the Eye of Odin from the Archmage’s brow, the Grimorum self-destructed, destroying the Archmage along with it.