The Four Heroes, otherwise known as the Heroes of Light, are the four main characters in Super Paper Mario. These four heroes are described as the Man in Red (Mario), the Fair Maiden (Princess Peach), the Evil King (Bowser) and the Man in Green (Luigi). These four need to collect the eight Pure Hearts, fragments of the Purity Heart, and stop the wielder of the Chaos Heart in order to save the universe.
It is read in the Dark Prognosticus that the Chaos Heart will appear if the fair maiden and the evil king marry each other. This is done in the game by Nastasia brainwashing Peach into marrying Bowser. Bowser was fine with the marriage. Once they got married, the Chaos Heart appeared and was taken by Count Bleck. With the Chaos Heart, Count Bleck was able to open The Void under his control.
The Light Prognosticus was written in order to counter-act the Dark Prognosticus. This book instructs the heroes to collect the eight Pure Hearts in order to stop The Void from consuming all universes. The Dark Prognosticus read "Though the man in red will wield formidable powers, one shall stand against him. The man in green shall use the Chaos Heart's power to bring darkness to all." In Super Paper Mario, this means that Mario, the man in red, has great power. But, when Dimentio controls the Chaos Heart, he brainwashes Luigi (the man in green) with a Floro Sprout. Luigi is used as a vessel for the Chaos Heart, combined with Dimentio creates Super Dimentio. Super Dimentio was supposed to bring darkness to all until the other Heroes of Light, Blumiere and Tippi stopped him.