Chestnut King

According to Luigi's "story" in Paper Mario: The Thousand-Year Door, the Chestnut King is the villain who kidnaps Princess Éclair from the Waffle Kingdom and brings her to Hatesong Tower.

Super Luigi plot
Luigi travels to the tower by using the Marvelous Compass. Once there, Luigi challenged the Chestnut King to an epic battle. The beast king, who was dripping with toxic goo, eventually lost to Luigi and his Hammer. However, before Luigi could deliver the final blow, Princess Éclair appeared and stopped him. As it turns out, the Chestnut King and the princess were actually lovers (much to Luigi's dismay). The King was transformed into a monster through the magic of Minister Crepe of the Waffle Kingdom. Eventually, Luigi defeats the shadowy Crepe, presumably restoring the Chestnut King to his former self. Luigi never admits that he was wrong in believing that the Chestnut King was the Princess's captor.

It is interesting to note that the king is actually the same as the Goomba King in Japan, however this was lost in translation.