One day, King Arthur was bewitched and became corrupt when the power of immortality was bestowed upon him by his scabbard, becoming the Black Knight. He has strong relations with the Knights of the Round Table, as they still serve him despite his change. His powers include using his blade, Deathcalibur, to summon Underworld minions to do his bidding and firing magical bolts on top of his impressive sword skills. His scabbard grants him immortality, making him impossible to defeat without obtaining the three blades held by the Knights of the Round Table. He is first seen chasing Merlina right before she summons Sonic. Sonic prepares to fight the Black Knight (not knowing he couldn't kill him) before being dragged away by Merlina. Sonic faces the Black Knight twice during the game. The first time is shortly after obtaining Caliburn the Sword, but lacking both the swords and the skill, he is unable to defeat him and the Black Knight leaves after deeming Sonic not worth his time. The second time is a more heated duel at Castle Avalon (the Black Knight apparently taking Sonic seriously this time) where Sonic is able to use the Knight's swords to nullify the scabbard's power and defeat him.
After being bested by Sonic, the Black Knight fades away, leaving nothing but his scabbard. When Sonic brings the scabbard to Merlina, she reveals that King Arthur was never there, just an illusion created by Merlin, and it was merely part of her plan to acquire it for her own ends.
Despite the fact that he is also first seen as King Arthur by everyone's eyes, Sonic's blade Caliburn reveals that since he is a legendary sacred sword, his duty is to decide who is fit for the crown. So that would technically mean that Sonic was the REAL King Arthur the entire time.
His theme song is "Fight the Knight", a song by Crush 40 for the two boss fights Sonic has with him. In the 1st boss battle against him, the song is instrumental. 2nd boss battle against him has lyrics.
He is also unlockable in battle mode by collecting the five books detailing the story of King Arthur during regular gameplay.