The phrase "when supernatural battles became commonplace" prominently refers to a Japanese light novel, manga, and anime series titled "When Supernatural Battles Became Commonplace" (Inō-Batoru wa Nichijō-kei no Naka de), which debuted as a light novel series in 2012. The series features a group of high school students who suddenly awaken to supernatural powers and face the reality that, despite their powers, their everyday lives remain ordinary with no impending battles or villains initially. The storyline explores their discovery and use of these powers over time and the eventual onset of battles, known as the "Fairy War," involving superpowered individuals. The light novel ran from 2012 to 2018, and the anime adaptation aired in 2014. In a more general cultural or literary sense, supernatural battles have been a thematic element in folklore, mythology, and later fantasy literature for centuries, but the expression in the query aligns with this specific modern work that literally addresses the concept of supernatural battles becoming a part of everyday life in a fictional setting.
If the question targets the historical or literary origin of supernatural battles becoming a common theme, that's a broader topic, but in a contemporary and popular culture context, this series is a notable specific reference.
