Simultaneously cheese, hat, and sword, this presumably-magical item was briefly described in an antique shop in Book 11. Fans of cheese, hats, and swords rejoiced at the combination of the three, and the Cheddarblade Cornuthaum is the subject of many fanfics suggesting origins and uses. As its name implies, it is a pointy hat made primarily of cheddar (specifically a hard variety called blade cheddar) which has an extensible handle so its sharp point can be used as a weapon.
Blade cheddar is renowned as the sharpest cheese in the world.
Cheddarblade Cornutham (Card) is also a card in the Adventure and Mysterious expansion.
Note: This hat (and all such blade cheddar) actually gains its powers from a little known school of magic known as Enrichment. This makes it legal, according to Item Law 54.