Complex sentence: She was a spriggan (secret treasure priestess) who lived in a town that rested underneath the Sky Library, guarding it; however, the Demon Lord from another world, Abyss Trinity, used the weapons of the Sky Library to destroy the city.
Split into simpler sentences: She was a spriggan (secret treasure priestess) who lived in a town that rested underneath the Sky Library, guarding it. However, the Demon Lord from another world, Abyss Trinity, used the weapons of the Sky Library to destroy the city.