Galbatorix, driven mad by the loss of Jarnunvösk, was denied another egg. As a result, he came to the belief that the Riders were the reason his dragon and partner in life was slain. Thus he plotted his revenge. He killed an elder, slew his erstwhile ally and fled after taking his slain ally's Eldunarí. In the wild he broke the will of the Eldunarí and gained Morzan's support.
With Morzan's help, he snuck into Ilirea where he slew another new Rider and stole his newly hatched dragon. Using dark arts, the two twisted the new dragon, Shruikan, to become Galbatorix's new steed. For the next several years, the two waited in the wilds as the dragon grew in size and power. During the intervening time, the two worked to gather followers and plotted to dominate Alagaësia.
The Forsworn, a group of disenchanted and power-hungry Riders, joined Galbatorix's cause. Hunting down wild dragons for their Eldunarí, the group worked to increase their power and ability to work magic. Slaying all the Riders and Dragons they encountered, the group methodically built up their stores of energy to topple the Dragon Riders.
The Battle Edit
The Forsworn descended upon the island, bolstered by the strength of the dozens of Eldunarí at their disposal. With the support of the Eldunarí they had captured, the Forsworn were more powerful than most of the Riders on the island and despite their numbers, the Riders were slowly but surely defeated.
Galbatorix slew Vrael's dragon, Umaroth, or rather the dragon's body as his Eldunarí had been hidden inside the Vault of Souls. Saphira I, Brom's dragon perished in the fighting and Undbitr, Brom's sword was lost as well. Countless Riders were killed both on the ground and in the air, whether they fought on Dragonback or not. Vrael, the mightiest of the Riders dueled with Galbatorix himself. Fighting with both mind and swords, the two fought throughout the battle until Vrael bested Galbatorix. But Vrael hesitated before striking his defeated opponent, who took the opportunity to smote him on the side. Grievously wounded, Vrael fled the island for Utgard Mountain.
Belgabad and the wild dragons, without the ability to control magic themselves fared little better. They too were killed in large numbers by the Forsworn. Many of the mothers who had hatched eggs in the Vault of Souls perished in the desperate fight. But even with their tremendous might, they were defeated, or else they were enslaved in the form of their Eldunarí.
Thuviel, an elven Rider who had lost his dragon only a month prior to the fight, cast a powerful spell in a desperate bid to kill the Forsworn. Half driven to insanity, he remained loyal to the Riders and sought release though he was desperately needed for the fight. By converting his body into raw energy, he caused a release of energy which destroyed himself in a powerful explosion akin to an antimatter detonation.
Most of the Forsworn had some warning and survived but Glaerun died in the explosion. Of the Riders, many were not as fortunate and thus they perished. Only a mere handful of Riders survived the fighting, explosion, and subsequent poison. Of those, Vrael and Brom are the only two of note.
Thuviel's Legacy Edit
The island was rendered uninhabitable due to an invisible poison. Though this was a side-effect of the spell, it was a failsafe plan devised in secret by Oromis, Vrael, Umaroth, Glaedr, and others who created the Vault of Souls. It was to ensure that the Forsworn would not seek to inhabit the island and so accidentally discover the vault, which held the secret hope of the Dragons and those opposed to Galbatorix.
It was this sacrifice that would ultimately allow the Eldunarí secreted away in the Vault of Souls to use their abilities to affect events across Alagaësia and help bring about Galbatorix's eventual downfall. It also ensured that the Dragons would survive as a race, restore the Riders, and finally bring a true peace upon the conclusion of the Rider War over a century later.