Succoru

The source of the corruption in the Realm of Light are the Succoru parasites, creatures of darkness that hatch from small seeds. It is unknown how they entered the Realm of Light, but their presence has led to numerous section of the region becoming uninhabitable for the native Heliolux, which are easily corrupted by contact with the affected regions. The main places that they appear in, are an abandoned Heliolux temple, and a damaged taproot. Larger versions of these creatures were in the process of being hatched but they were killed by the player during the quest The Realm of Light. It is possible that the damage they did to the taproot is part of what created and sustains the power of the Decay that has spread across much of Northern Gavel. Because they are creatures of darkness, their contact with the light allows them to corrupt the surrounding areas. This is why the player, as a human is not as greatly affected by proximity to the creatures, as they are not made of pure light as the Heliolux are.

As said by Heliorion, another parasitic creature possibly related to the Succoru would break into the Realm of Light several decades before the player's arrival and attach itself to Orphion, the beast of light. This parasite is not mentioned to have actually corrupted Orphion, but instead leeched off of his power, making the light beast sick and eventually lose control after many years.

Sourcing

 * For more information on how light and darkness interact, see Aldorei's Secret Part II, and Wynn Excavation Site D.
 * Additionally, the quest Taproot, and Finding the Light offer more info about the problems face by both Gavel, and the Realm of Light.