In Tokyo Ravens, when Kagami Reiji is dealing with the Nue when he makes his first appearance, he utters the syllable "un" (ウン), which harms the Nue in some way. In the manga, this is displayed as follows:
On the right we have furigana that read "un" (ウン), but the symbol on the left is clearly not Japanese. What's going on here?
In Shingon Buddhism, there is a concept known as shuji (種字・種子; Sanskrit bīja or bījākṣara), which means "seed syllable". These are monosyllabic Sanskrit-derived incantations (i.e. mantras) which are taken to hold mystical power in and of themselves, and which symbolize a particular revered figure (e.g. a Buddha or bodhisattva).
The symbol that Kagami is shown uttering appears to be a stylized version of the seed syllable "हुं", transliterated as "hum", which finds its way into Japanese as "un". This particular one symbolizes Kundali (軍荼利明王 gundari myouou), who is one of the five Wisdom Kings.