Cain's Old Journal is a manuscript divided into two parts in Diablo III, both of which are found in the first act of the game. Each part can be found in an Old Keepsake Box along the Old Tristram Road or in The Old Ruins. Both parts add to A Quick Study achievement.
The parts read as follows:
I can no longer deny the true nature of the evil that rises from the depths of the cathedral. It is almost too horrible to admit the truth of it. It is the dark Lord of Terror, Diablo himself, who plagues us.
The vile staff of Lazarus was brought to me today, confirming my suspicions. There is no longer any doubt that Lazarus kidnapped Albrecht and perhaps even freed Diablo from his ancient prison. Who knows what further treachery he has planned?