The True Revelation is a relic that can be found in The Darkness II. It is an attempt to decipher "The Revelation of John", the last book of the Bible.
According to Johnny Powell, there are three different interpretations of the Revelation of John: as a literal prophecy, which he dismisses as retarded; as a metaphor for Roman persecution, which he says is "plausible, but boring"; and as a code that can reveal the True Revelation when properly deciphered. He adds that adherents of the third interpretation tend to be crazy, but fun at parties.
The people who believe that the Revelation of John hides the True Revelation decipher it differently, which is why each copy of the True Revelation is unique. This particular copy was written by the Brotherhood and predicts their fall into madness.