King Minos would send seven women and seven men every year into a maze that has a deadly creature called a Minotaur.
Theseus knew that this was bad, so he went to fight the Minotaur on a ship with black sails.
Theseus father did not want his only son to die, so Theseus told his father if his ship comes back with black sails the he died, but if it comes back with white sail he has survived and defeated the Minotaur.
King Theseus went into the maze to fight the Minotaur with just a sword to slay and string to make his way back.
Theseus defeated the Minotaur
Theseus forgot to change the sails on the ship, and when King Aegeus saw the black sails he thought his only son has died. So in a lot of grief he threw himself of a cliff thinking his son had dies.