The Book of Death is one of two extremely powerful books in Neon White; at the beginning of the game, it is held by the Believers.
Story[]
The Book of Death is in the hands of the Believers at the beginning of the game; they use it primarily as a tool with which to control the Neons. At some point in the past, they used it to forge the Mechanical Halo, in the possession of Neon Green.
During Chapter 10, White breaks Green's Halo, freeing him from the control of the Believers; soon afterwards, Green steals the Book of Death from the Believers and goes on a rampage through Central Heaven, killing nearly all Neons and all Believers. He holds onto this book until the end of the game in Chapter 12, wherein White defeats Green in a duel and faces a choice: write Green into the Book of Death with the Inkhorn, or write him into the Book of Life.
If White writes Green into the Book of Death, Green is destroyed, though God will write White too into the Book of Death if he does this.
History[]
The Book of Death once belonged to God, and was used during the Ten Days of Judgement to write the souls of sinners who had refused to learn or grow from sinful and heinous actions in life. Those written into the Book of Death during those ten days would appear in Heaven as Demons.
It is assumed that the Believers stole this book when they overthrew God at The Third Temple; its original resting place was the clock tower in The Old City.