Light is introduced in Golden Sun: Dark Dawn as one of the two yin-and-yang-like values that help bring existence into being, along with Darkness, separate from the four great Elements that comprise all physical matter - Venus, Mars, Jupiter, and Mercury. It is not certain whether this qualifies as an Element in and of itself, but the concept of employing Light as a form of Psynergy like a normal element is briefly touched upon at the end of Dark Dawn.
Role in Story
Light-aligned abilities are not common knowledge to Weyard's present-day inhabitants, but were discovered in the ancient past. Having mastered the four known elements, the world's ancient inhabitants sought further control over alchemy, a quest Kraden would later describe as a search for nothing short of omnipotence. To this end, they built the Eclipse Tower, a device which absorbed light. An unexpected side-effect is that a huge area of darkness was created across Weyard, known as the Grave Eclipse. Eclipse Tower was later sealed and deactivated, until the events of Dark Dawn.
As a result of the ancient Grave Eclipse, the ancients created an Alchemy Machine known as the Apollo Lens, which possessed the ability to gather and concentrate Light into a powerful beam which, when aimed at Eclipse Tower, could shut off the tower and end the eclipse.
Near the end of Dark Dawn, Matthew's party of Adepts activate the Apollo Lens to end the current Grave Eclipse. As the Apollo Lens is fired at Eclipse Tower, now in the town of Belinsk, light energy concentrates over the town. The town's beastman denizens later report that they were endowed with the ability to project light as Psynergy when they were caught in the light. These beastmen's fur color is changed to bright yellow to reflect this, as shown by Vande.