花吹雪く王, or "sovereign of the flower storm" in English, is the title of Danan, the ruler of the elves. It is an interesting phrase in more than one ways. In Japanese, 花吹雪 (HANAFUBUKI) is an expression that refers specifically to heavily falling cherry blossoms. Japan is renowned throughout the world for its cherry blossom season (in Spring). It's composed of the words 花 (HANA), meaning flower, and 吹雪 (FUBUKI), which by itself refers to a snow storm.
花吹雪 (HANAFUBUKI) is a noun, but in this case, Miura used furigana and a く hiragana to change it into a verb pronounced HANAFUBUKU. This makes it a modifier for the last word, 王 (OU), which means "king". It isn't easy to adapt to another language, but Puella quickly proposed to translate it as "King of the Flower Storm" in March 2004. It remains the best English translation for it to this day.
However there is one wrinkle to it. In volume 39, twelve years after revealing the elf ruler's existence, we were finally introduced to her. It turns out Miura sensei had played a little trick on us, and the ruler of the elves was in fact a woman. Why not use the word "queen" then? Maybe to surprise the readers, maybe because he didn't feel the need to. Either way, as a result we changed "king" to a genderless synonym: "sovereign".