The Harry Potter series has gained substantial popularity ever since the first book was published in 1997. The series has proven interesting both from literary and linguistic points of view, but the idiomatic language found in the books has yet to be thoroughly investigated. This essay aims to present and analyse the modified idioms and proverbs found in the fourth iteration of the Harry Potter series, Harry Potter and the Goblet of Fire by J.K. Rowling, while also providing a comparison with the real English idiomatic expression. To do so, this study uses Sullivan’s (2001) theory of the secondary and primary world and the semantic relationship of hyponyms. Most of the studies on idiomatic language of the Harry Potter series are focused on the translation of modified proverbs and idioms, not on the modification of the proverbs or idioms themselves. By doing a thorough read of the fourth book I found eight idiomatic expressions with a real English counterpart. I then analysed these expressions and the modifications done by Rowling. The results from this study show that the idiomatic expressions used by Rowling are modified, but only to an extent, as to keep them recognizable to the reader. In most cases the only modification is that to the nominal phrases. Neither the syntactical structure, nor the grammatical form changed very much. The results from this essay show that the changes to the expressions were mostly semantical changes.