The Gardiner Grimoire is an ancient book of spells which has been passing down through generations of the Gardiner family. It was left by Penelope in Fair Haven for her two sons to find. It was hidden under a floorboard in the attic of the mansion. Dash and Killian Gardiner, afraid of what it did to Killian, put it back in its hiding place.
The Gardiner Grimoire was hidden under a floorboard and kept wrapped in leather. It has two covers: one apparent, made of clear wood and which bears a strange symbol onto it (which could be a scorpio, a horned beast or something else) and one hidden behind, made of traditional red leather.
Though the Gardiner Grimoire is similar in use to the Beauchamp Grimoire, it appears that in nature, its content is far more versed into dark and harmful magic. After trying a spell which held disastrous consequences, the Gardiner brothers decided to put it back where it came from.