Bjørn, known as Bjørn the Behemoose, is a formidable monster that terrorizes the town of Mostroferrato. The creature steals the Zenithian Sword from the wealthy nobleman Rodrigo Briscoletti, who in return offers his daughter Nera’s hand in marriage to whoever can defeat it.

The protagonist Luca, seeking the sword to prove himself the legendary hero, accepts the challenge with the aid of his childhood friend Bianca. They confront the Behemoose and retrieve the sword, though Luca’s inability to wield it reveals he is not the prophesized hero. Forced to flee after awakening the creature, they later identify a weakness in Bjørn’s defenses. Luca strikes this vulnerable point with Bianca’s support, causing the Behemoose to yield—a moment marked by the fading of its orange eyes. Luca then spares Bjørn’s life.

After its defeat, Bjørn becomes an ally. In the final battle against Bishop Ladja’s forces, Bjørn arrives alongside allies such as Sancho and Prince Harry to aid Luca and his family when they are overwhelmed. This intervention shifts the battle’s momentum, enabling Luca to confront Ladja directly.