There have been many kings who were unfit to rule and a few of them were completely insane. Two prominent examples of mad rulers are Henry VI of England and Charles VI of France. Henry VI was a pious man, but a weak, indecisive, and insane king. His unstable rule and inability to reign under his own power was one of the main factors that led to the War of the Roses. Henry VI’s grandfather, Charles VI was so crazy that he thought he was made of glass and about to break. He even had his servants stitch iron rods into his clothing to keep him from shattering. Charles even forgot he had a family from time to time and had unpredictable mood swings that resulted in him murdering some of his servants. In my fantasy series, there is a mad Emperor known as Caelum II and I based him on both Henry VI and Charles VI. Caelum II was so weak and indecisive that he was unable to hold his Empire together when the Dark Death plague ravaged it. House Sylva used the resulting chaos as a pretext to seize the Imperial Throne from Caelum, which resulted in House Sylva serving as the new Imperial Dynasty for three hundred years. You will see his madness in closer detail in a flashback in the second book.