The Ivory Door A Legend in a Prologue and Three Acts

From Wikipedia: Act 1 has the old king working alone in his private room when his young son, Perival, enters. After the two discuss such subjects as love, marriage, governing the kingdom, and the inevitability of death, the conversation turns to a door behind a tapestry. Perival says there are rumors that anyone who walks through the ivory door will be killed by the demons in the passage inside. The king says that he does not know whether that is true or false, because he has never been through the door and does not know anyone who has. He shows Perival the door and warns his son to not tamper with things beyond his understanding when he becomes king.