Everblight is one of the dragon "children" of Toruk who rebelled against him at their birth millennia ago. Slightly weaker than his brethren but more cunning, he worked in the shadows until things changed forever several centuries ago. At that time his hiding place under an Iosan city was discovered and the elves destroyed his corporeal form and imprisoned his athanc (a crystal heartstone containing his essence) on a desolate mountaintop in north Khador. Decades later he was freed but instead of returning to a corporeal form he has split his essence and mastered the effects of his blight. Now the northern tribes of Ogrun and Nyss elves are fully under his sway and comprise his Legion. They march to war alongside powerful dragonspawn. The Legion is a small but rapidly growing force intent on spreading Everblight's control across all Immoren.
For more backstory, refer to Lore - Everblight