Glaerun

Glaerun was an elven Dragon Rider, bonded to a dragon of unknown colour, who joined the Riders before the Fall. Due to a perceived injustice against him by the Riders, however, Glaerun and his dragon betrayed their order to join the human Rider Galbatorix and became members of the Thirteen Forsworn sometime during the rebellion, assisting him in his violent campaign against the Riders, presumably including the slaughtering of Riders and Dragons, tame and wild alike. In the process of the war, however, all of the other dragons joined together and stripped the Forsworn's dragons of their names, destroying Glaerun's dragon's sense of identity.

With Galbatorix and the other Wyrdfell, Glaerun eventually flew on Vroengard, engaging in battle with near-every remaining member of the order. In the fray, however, Glaerun and his dragon were slain, consumed by an explosion caused by the elf Thuviel, marking Glaerun as the Forsworn's first casualty.

Early Life and Training
Presumably upon or after reaching the age of 20, Glaerun, as was the custom, was chosen by a dragon hatchling and gained the gedwëy ignasia upon whichever palm touched the egg that hatched to him.

After this choosing, Glaerun and his dragon were taken to the elven capital of Ilirea, where he was apprenticed to an unknown elder pair and trained in aerial maneuvers (especially those used in battle), building stamina for long-distance flights, increasing speed, dragon lore and the ancient language. Also, as was a mandatory requirement, Glaerun and his dragon would share their lessons, keeping their minds connected so they could listen in on each other's lessons and learn, then were tested by their teacher to ensure that both learned each other's subject. As was typical, the young Glaerun was surely given a set of frustrating, apparently impossible tasks that could not be accomplished without the aid of magic until his frustration gave way to magic and he began his arcane training. He was taught furthermore to install mental links with creatures other than his dragon and surely became a master at this art of reading others, as well as mentally fighting others via their mind.

At the end of the training, the Elven smith Rhunön forged a sword for Glaerun, as was typical, which Glaerun continued to wield for the rest of his life.

Joining the Thirteen
During the time of chaos when Galbatorix was amassing power and followers, Glaerun and his dragon partner betrayed their order and races to join the mentally unhinged Rider in his campaign against the Riders. Though the reasons behind Glaerun's treachery are unexplained, he might have been motivated by a hunger for vengeance against a past wrong wrought on him, a belief that the powers of Riders and dragons entitled them to rule all of Alagaësia or even a simple desire to tear down what his predecessors had built and indulge himself however he wished. With twelve other traitors and their dragon partners, Glaerun and his own dragon formed the 13 Forsworn and set out to topple the Order, killing any Rider they met and slaughtering dragons both tame and wild alike with the objective of seizing as many Eldunarí as they could.

With Galbatorix and the other 12, Glaerun flew to the plains of Du Völlar Eldrvarya, where he fought in the First Battle of the Burning Plains, fighting against the Dragon Riders during which the flames the dragons (including Glaerun's) of both sides used to attack their foes permanently scarred the landscape. These flames also lit the vast deposits of peat in the area, and the fire apparently "burrowed" underground. Consequently, there are frequent bursts of flame coming out of the ground even to this day. The outcome of this battle is unknown.

In the conflict between the Shur'tugal and Wyrdfell, Glaerun's Dragon, as punishment for his or her treachery, was a victim of the Banishing of Names, where the entire Dragon race (bar the twisted Thirteen) stripped the Thirteen of their names, and through them, their identities and ability to define themselves as intelligent creatures, though it is unclear how being able to do nothing but watch in misery as his partner of heart and mind deteriorated into ignorance.

Glaerun was also present in the Forsworn's attack on a almost unguarded Ilirea (most of the riders had gone to defend Vroengard). Despite being a force made out of only fourteen (twenty-eight considering the Forsworn's twisted dragons and Shruiken) against an army of humans, elves, dwarves and Dragons, Galbatorix and Forsworn held their ground, slaughtering the fighters of the three races of human, elf and dwarf. In particular, Glaerun and his comrades were responsible for wiping out nearly every warrior of the clan of Grimstborith Anhûin. Finally, despite the best efforts of the combined forces of the riders, dwarves and elves, the Forsworn eventually ended victories, due to the large amount of Eldunarí they had seized from dead riders, their dragons and the Eldunarí the dead riders also held.

Death
Finally, having gained enough Eldunarí to face of against that full for of the riders, Glaerun, Galbatorix and the rest of the Forsworn flew on island of Vroengard, where they laid siege to the Riders at Doru Araeba, intent on destroying them. During the immense battle, one of Glaerun's enemies, an elven rider by the name of Thuviel, who had lost his dragon to the hands of a member of the Forsworn (perhaps Glaerun himself) 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 to the end 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. Perhaps to embroiled in fighting to take notice of the moment of warning his fellow oath-breakers had, Glaerun and his dragon thus lacked the necessary defences the other Wyrdfell had thrown up and were consumed by Thuviel's explosion and killed, marking them out as the first death the Forsworn suffered.

Powers

Since Glaerun has not made a physical appearance in the books, the extent of his powers cannot be certain. It is likely that he was very skilled at swordplay, mental powers and magic, since he was a Dragon Rider and slew many others of the order, as well as many warriors of the elves, dwarves and humans and was likely well-versed in using the Twelve Words of Death. He also learned dark magic from Galbatorix and possibly drew power from some Eldunarí he captured during the fall of the riders. Paolini stated that Galbatorix eventually gathered most if not all of the Forsworn's Eldunarí for himself but since Glaerun died from Thuviel's explosion it is unknown if the Eldunari were recovered from his body or dragon's body.