Vroengard was an island located off the coast of northern Alagaësia. It was home to Doru Araeba, the city of the Dragon Riders. Among the ruins of Doru Araeba the Rock of Kuthian, the magically sealed entrance to the Vault of Souls, was located.
Before the Fall of the Dragon Riders, the island was the home of the Dragon Riders. The city of Doru Araeba, located in the east of Vroengard, was their capital.
It was also the site of the last battle of the fight between Galbatorix and The Forsworn and the Riders where Vrael, the leader of the Dragon Riders, made his last stand in battle against Galbatorix, before his flight to Utgard Mountain. During this battle, a Dragon Rider named Thuviel performed a spell on himself, apparently as a last-ditch attempt to destroy the attackers, in which he converted every atom in his body into pure energy, most likely by the process of nuclear fission. The spell killed Thuviel and resulted in a blast of cataclysmic proportions, decimating the island but failing to kill Galbatorix and all but one of the Forsworn, who shielded themselves. The Forsworn Glaerun and his Dragon were both killed, as were many Riders and Dragons, both wild and bonded. The explosion also resulted in the entire island being contaminated with an "invisible poison," which is probably radiation, given the nature of the effects caused by it, such as skin lesions. Glaedr, however, did not know this word to describe it (the concept being beyond the understanding of the science of the time) and so simply stated it to be an "invisible poison."
As the energy of nuclear fission can only be released by the mass-to-energy conversion of larger atoms, namely, those of atomic numbers greater than Iron in the periodic table, it is clear that the Riders would be unable to liberate any energy from the organic molecules of their body. They would therefore be unable to create an explosion as breaking the bonds within the atoms in their body would require energy. Even if they managed it, they would be unable to generate any radioactive elements with appreciable half-lives, as this requires higher mass elements suspended in the atmosphere. This is speculated, therefore, to be a result of magic.
A second method by which high energy can be released is by particle annihilation, essentially every proton, electron and neutron are turned into high frequency electromagnetic waves by colliding them (assumingly by way of magic) with their anti particles, or by completely converting the particles into the energy represented by e=mc^2, which is their kinetic relative energy. This matches better the spell uttered later by Galbatorix: "Be not!".
The island was home to several varieties of unique wildlife. Notable among these were the snalglí, a species of gigantic snail that the Dragons enjoyed eating for food. After the blast of energy, caused by a rider splitting the atoms of his body, the poison (a.k.a. radiation) in the island corrupted much of the wildlife, causing them to adopt unusual physical and magical characteristics, such as the Burrow Grub.