Glaerun was an elven Dragon Rider, bonded to a dragon of unknown color, who joined Galbatorix and became one of the Forsworn. He lived at the time of the Fall of the Dragon Riders, but due to a perceived injustice against him by the Riders, he betrayed his order to join the Forsworn. He was the first to die of the Forsworn, consumed by Thuviel's explosion on Vroengard.
In an interview with Random Buzzers in December 2011, Christopher Paolini revealed that Glaerun was an elf.
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. 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 he died from Thuviel's explosion it is unknown if the Eldunari were recovered from his body or dragon's body.