The Waffle Kingdom is the main location of Luigi's story in Paper Mario: The Thousand-Year Door.
Luigi was summoned to the Waffle Kingdom by Crepe to help save Princess Eclair from the Chestnut King. Luigi ventured though different locations collecting pieces of the Marvelous Compass to help track down the Chestnut King's location. Luigi eventually does and defeats the Chestnut King and combat. However, it was all a trick by Crepe who attempted to take over the world now that the Chestnut King is defeated. However, using the power of the Marvelous Compass, Luigi was able to defeat Crepe and save the Waffle Kingdom. The Chestnut King and Princess Eclair had marry and Luigi, disappointed, heads back to Rogueport to tell Mario his story (though he doesn't mention the part about Crepe being the true villain and the Chestnut King and Princess Eclair getting married).