Readers would assert that J. R. R. Tolkien was the popularization of the fantasy genre, with his hugely successful publications – The Hobbit, and The Lord of the Rings. Tolkien himself, though, was largely informed by an ancient body of Anglo-Saxon myths — particularly Beowulf — as well as modern works such as The Worm Ouroboros by E. R. Eddison, but it was after his work that the genre began to receive the moniker, "fantasy" (often applied retro-actively to the works of Eddison, Carroll, Howard, et. al). J. R. R. Tolkien's close friend C. S. Lewis, author of the The Chronicles of Narnia, also an English professor interested in similar themes, was also associated with popularizing the fantasy genre.
Ursula K. LeGuin, in her influential essay, "From Elfland to Poughkeepsie", criticized the use of a formal, "olden-day" style for writing high fantasy. While she admired the archaic style for its ability to distance prose into a fantasy world rather than appear as a modern world in disguise, when it was used by masters such as Lord Dunsany and E. R. Eddison, she also noted that it was a dangerous trap for fantasy writers because it was ridiculous when done wrong. Michael Moorcock observed that many writers would use archaic language for its sonority and to lend color to a lifeless story.
The fantasy world requires, like any genre, appropriate language, and that language can vary. In various forms of fairytale fantasy, even the villain's language would be inappropriate if vulgar.