In Willaca's continuity. King Arthur is the Leader of the Dark Army and was the one who forced Dr. Eggman to kill Will's father. Although Will doesn't know it.
Sonic Nega, the newBlack Knight
One day, while Sonic Nega was running from Captain Whiskers, he tripped on King Arthur The Black Knight's sword, The Deathcalibur. He picked it up, and all of The Underworld King's memories came flowing thru him, and he found himself senselessly beating Mini and Mum into the dirt, with damaged Robo-Pirates running from him. Then, he saw The King himself, and Arthur said: "You have found my blade, I see. I now dub thy, the New King of the Underworld!", Arthur charged at Nega, and then entered Sonic Nega's body, allowing Nega to summon Knights of the Underworld, and become a male version of The Dark Queen, called The Underking. But, with the Scaburn's dark magic, he became partly corrupted.