First you must unlock the Broadsword by redeeming Chalices in the Hall of Heroes. It's given to you after redeeming 5 chalices (2nd statue on the left in Hall of Heroes).

Once you got the weapon, go to any merchant gargoyle and buy the Enchantment for the Broad Sword. This works at any merchant gargoyle. A quick one to find is at the start of "Scarecrow Fields" after kicking the first cart in that level. The enchantment costs 100 coins.