The story about the statue is, that one night in thirteenhunderd-something, Pedro, king of Castille, walked through the streets of Seville. He had an encounter with Louis Guzman, a member of a family with influence. This family supported the bastard brother of King Pedro, Henry of Trastámara, in his aspiration of the thrown. So the Guzmans and Pedro were enemies. They got into a fight and Pedro killed Louis. An old lady was the unfortunate witness of the killing from her window. She recognized the King by the sound of his knees [as a child he suffered from slight braindamage which resulted in a strange sound as he walked]. When the old lady closed her window in haste, she dropped her lamp, next to the dead Louis Guzman.
When the Guzmans came to find justice for their killed son, Pedro swore the killer would be decapitated and his head would be exposed on the scene of the crime. Because of the lost lamp, the old lady was traced as a witness, but she wouldn't tell what she had seen [Pedro was not called 'El cruel' for nothing]. So when Pedro assured her nothing was going to happen to her, she held a mirror to the King and said: "Here is your killer".
The next day Pedro brought a closed crate, in which, he claimed, was the head of the killer. The crate was placed into an alcove at the place of the killing. Pedro summoned the crate would not be opened until after his death.
His death came on the battlefield with his brother Henry. He was captured and … beheaded.
With which he kept one part of his oath to the Guzmans.
When the Guzman family heard of his death, they opened the crate to find a bust of Pedro in clay. The bust was put on the wall to be replaced later with the one we can see until today.