Part One (Trollhunters)
Toby, Blinky, NotEnrique and AAARRRGGHH!!! came across the Book of Ga-Huel after infiltrating Stricklander's hidden vault at Arcadia Oaks High. Taking a peek through its pages, Toby notes an illustration of Gunmar on one page and a vicious Gumm-Gumm that resembled AAARRRGGHH!!! on the other. NotEnrique asserted that is was indeed AAARRRGGHH!!! and a sigh from the gentle giant confirmed the horrible truth. Blinky then told Toby of how AAARRRGGHH!!! was a general in Gunmar's horde, but later changed sides before the Battle of Killahead. Taking another look at the illustration of AAARRRGGHH!!!'s former self, Toby insisted that it was all in a past and that his wingman was not the monster he once was.
During the Spring Fling dance, Claire and Toby infiltrate Strickler's office again to get a page with the counterspell required to sever the magical bond between Strickler and Barbara Lake. Due to the unexpected interference of the teachers and Angor Rot, Claire is forced to throw her Shadow Staff through the portal right into Trollmarket.
Part Two (Trollhunters)
The Book of Ga-Huel plays a significant role in the novel The Book of Ga-Huel.
Part Three (Trollhunters)
Strickler brought the Book of Ga-Huel with him to Claire's house after the Trollhunters discover she was possessed by the spirit of Morgan le Fay. Blinky expressed concern over using a book full of dark magic and Strickler, stating that all dark magics come with a price, concurred. Holding open the book, he showed an illustration that revealed that Morgana was using Claire as a vessel into their plane of reality. Subsequently discovering that she possessed no shadow, Strickler realized that Claire's spirit was trapped in the Shadow Realm and their only hope was traveling into the space between realms to save her before Morgana retains control of her body permanently.