The book

Honkai Impact 3rd is set in an alternate version of Earth, plagued by catastrophes resulting from "Honkai" (roughly translates to "the collapse"). Honkai is largely viewed as a malevolent force with a will of its own, able to corrupt humans into undead creatures, manifest itself in the form of various monsters, and select various individuals known as "Herrschers" to imbue with god-like powers able to trigger apocalyptic events. The Honkai is a cyclic phenomenon, returning to end civilization on Earth each time it gets too advanced - the Previous Era ended 50,000 years prior to the game's start, after the 14th Herrscher defeated humanity.