The Light Prognosticus is a book in Super Paper Mario. This book was written by Merloo (or Merlumina, both are said to have written it) in order to counter-act the Dark Prognosticus. The Tribe of Ancients discovered the Dark Prognosticus long ago and discovered what tragedies would be in store if it was correct. So, the Tribe of Ancients decided to make up a plan in case all of the worlds would end, as predicted in the Dark Prognosticus.
The Dark Prognosticus read that someone would marry a maiden princess and an evil king, creating the Chaos Heart. The wielder of the Chaos Heart would then create The Void, a black hole that would gradually grow until every universe is destroyed. When all of the universes were destroyed, the wielder of the Chaos Heart could create the entire universe in their vision. Once the Chaos Heart would be stopped, the Void would close. The Tribe of Ancients, being a very loving people, thought that love would be the only thing to stop the Chaos Heart.
So, Merlumina created the Purity Heart. The Purity Heart was the very embodiment of love, a love force strong enough to stop the Chaos Heart and the Void if it ever should appear. The Purity Heart can be recharged by love when used, so it can be used by those with the power of love. That way, they thought only heroes could use it's power. As a precaution so that the Purity Heart wouldn't be stolen, Merlumina divided the Purity Heart into eight Pure Hearts. These eight items were taken by Tribe of Ancient members and were hidden in eight different dimensions. Once united, the Pure Hearts could form the Purity Heart again.
The Tribe of Ancients needed a way for the Heroes of Light, the heroes that were destined to stop the Chaos Heart, to know what to do. That is one reason why the Light Prognosticus was written. This book would instruct the heroes on how to find the Pure Hearts and how to stop the Dark Prognosticus from coming true. The other reason why the Light Prognosticus was written was to inspire hope. The The Four Sages (Merloo and Merlumina, the possible authors, being two of the sages) of the Tribe of Ancients wanted their civilians to believe the dimensions would never be destroyed.
Unfortunately, the Light Prognosticus isn't a prophecy like the Dark Prognosticus is. The Four Sages certainly hoped that the Light Prognosticus would come true, but it was written to counter-act the Dark Prognosticus. So, they just hoped that the Dark Prognosticus prophecy would be wrong.
The Light Prognosticus was taken by the son of a magician in the Tribe of Ancients after the Pixl Rebellion. The Light Prognosticus (and the first Pure Heart) was passed down from generation to generation. The living relative in-game with the Light Prognosticus and the first Pure Heart is Merlon. Merlon tells Mario, the "man in red" of the Heroes of Light, what the Light Prognosticus means. The book can be seen in Merlon's house on a pedistal, but cannot be read.