The Book of Monsters was formerly a necromancer named Monstrox, who terrorized Knighton with his dark sorcery. Once having been a member of the Wizards' Council and ally to the likes of Merlok, he discovered the Forbidden Powers. The forbidden powers drove him mad and corrupted him as he experimented with dark magic. He then chose to use the forbidden powers and his dark magic to wreck havoc across the land. The Council was left with no other choice but to expel him from the council and end his reign of terror, and so Merlok set out to defeat Monstrox.
After Merlok had turned his original army into stone, shrinking his castle to be placed inside a snow globe, Monstrox vowed revenge against Merlok. The good wizard Merlok eventually defeated Monstrox by turning him into a book, dividing most of his evil power among eleven other spell books, believing that he could control him and his powers. Although he knew that destroying the spell books would ensure Monstrox's eternal defeat, Merlok was reluctant to sacrifice sources of such potent (if evil) magic. He took the books (including the former Monstrox) to his library, where they sat on a shelf for many years.
Following his defeat, Monstrox bided his time and plotted revenge against the Kingdom of Knighton. Knowing that he still had some power even in his book form, he waited years to find someone that he could manipulate into acting as his pawn. He found it in a young Jestro, dropping out of The Knight's Academy, voicing his insecurities and concerns with Merlok. Realizing he could use Jestro's insecurities against him he spent the next five or six years plotting.
His plan eventually came to fruition when Jestro found the Book of Monsters after wandering off after the embarrassment and humiliation of his failed performance at the Knight's graduation. Sensing a perfect pawn at last, the Book manipulated and coerced Jestro into becoming a "bad boy," promising him tremendous power that would stop anyone from laughing at him ever again. With the Book's power, Jestro summoned a lava monster army, including a Book Keeper to hold the Book of Monsters so Jestro wouldn't have to. They then planned out multiple attacks on the Kingdom of Knighton, but were pushed back and defeated.During the ensuing war, the Book of Monsters encouraged Jestro to seek out the other eleven spell books which contained his remaining power, explaining that he could perform new and stronger spells by consuming them. Despite the Nexo Knights' best efforts and a number of other setbacks (including Jestro himself returning to the side of good), the Book eventually succeeded, restoring Monstrox's full power. He promptly attempted to possess Clay, sending his army to delay the other knights when they arrived to save their comrade. The knights held their own against the lava monsters, but this only allowed Monstrox to reabsorb their energy, making him powerful enough to defeat them all in a single strike.
Just when all hope seemed lost, Clay was given the sword Techcalibur, containing Merlok's data, by the Black Knight mech, who allowed himself to be sucked into Monstrox's mouth. Overcome by the pure magic energy, the villain exploded spectacularly, taking all of his summoned monsters with him and leaving only the charred cover of his book form. The Kingdom of Knighton celebrated Monstrox's defeat, but Jestro saw the former necromancer's face in a distant cloud, hinting at his survival.Unbeknownst to the kingdom, Merlok's sacrifice only served to destroy the Book of Monsters' pages, the binding was preserved and Monstrox lived on as the evil spirit within endured in the form of a living storm cloud. Enraged at yet another failure, Monstrox changed his plans from conquering Knighton to destroying it outright and mind controlled Jestro into becoming evil again. He then set about reviving his original army of monsters by using his new lightning abilities to reanimate their petrified bodies, all the while planning to unleash the Stone Colossus.