With hunting requests declining due to one of the Devil King's generals moving in nearby, Kazuma accompanies Megumin as she practices her explosion magic on a seemingly abandoned castle. This ends up infuriating the general, Beldia the Dullahan, who had been living in that very castle. Placing a death curse upon Darkness, Beldia demands that Megumin come to his castle within a week in order to save Darkness' life. Shortly after Beldia takes his leave, however, Aqua simply removes the curse from Darkness, eliminating the need to make the journey to the castle.
In the Light Novel, Kazuma learned both "Basic Magic" and "One-hand Sword." In the anime, they only showed him using the "Create Water" magic from the Basic Magic skill.
In the LN, the Axel Guild knew and told its adventurers that Beldia had moved into an abandon castle nearby. In the anime, they just knew Beldia had moved to somewhere nearby, but didn't know his exact location.