The Knight Erring (騎士の不覚, Kishi no Fukaku) is the sixteenth chapter of the Fire Force manga series.
The strange looking 5th member explains Setsuo's new strength due to him intaking experimental drugs. As he reveals his new weapon placed on his arm, Setsuo begins his assault on Arthur. Managing to defend successfully from Setsuo's attacks, Arthur remarks himself as a knight. Princess Hibana, residing in a different location from the fight, asks for an update of what's going on between the two brigades, in which a 5th bodyguard informs her of the confrontation between the 3 Angels of the 5th and Hinawa, Arthur and Setsuo, and Akitaru is still no where to be found. Hibana states that she wants to find the secrets of the Human Body Ignition Phenomenon and that worms like the 8th don't deserve to know. Arthur becomes cornered while Setsuo hammers into him. Persistently trying to attack him, Arthur doesn't manage to land a hit, to which Setsuo declares that he is a weakling who stays calm even though they aren't fighting well. Arthur pauses to think and realises something has been wrong the whole time; Excalibur is held in his left-hand even though he is right-handed. As Setsuo charges up his final attack, in a flash, Arthur strikes him with Flash of Lightning, splitting the Flame Human in half, ending in a quick and unexpected defeat. The strange looking 5th member looks in amazement at Arthur, and Arthur swings his sword stating he knew something was off.