"I am a monster at the centre of a maze. I am half human and half beast. A hero killed me with a sword and a ball of thread. What am I?"

Answer: The Minotaur. In Greek mythology the Minotaur was a creature with the body of a man and the head of a bull, imprisoned in the Labyrinth on Crete. The hero Theseus navigated the maze using a thread given to him by Ariadne and killed the Minotaur.