The Soul of Grunhold is the first horn that protected the Grunhold, the domain of the Centaurs of Fablehaven. It was hidden at the end of the Tauran Maze, inside the Heart of Grunhold. Like all unicorn horns, the protective spells on it emit such guilt that even the most notorious thief could not successfully steal it.
In Secrets of the Dragon Sanctuary, Seth stole it for the Knights to open the gates of Wyrmroost. In Keys to the Demon Prison, it is revealed by Bracken to be his first horn. The horn is reclaimed by Bracken as punishment for the centaur uprising and replaced by a Spell. The spell is powerful but not nearly as potent as the horn was.