Formora was a female elf, bonded to a brown dragon, who joined the Dragon Riders at a young age. She was trained in the ways of the Riders and eventually joined Galbatorix on his quest to conquer Alagaësia. As such, she became one of the infamous Forsworn.
Not much is known about Formora's early history, but it is known that she was bonded to a brown dragon and trained in the way of the Dragon Riders by an unknown teacher.
In the time that Galbatorix was gaining power and followers, two senior members of the Order decided (partly at the urging of Formora and Kialandí, another of the Forsworn) to find out what they could about the whole situation for themselves. These were Oromis and his dragon partner, Glaedr.
Oromis and Glaedr took Formora and Kialandí with them on their venture and after a short time of flying, the treachery of the latter two was revealed. The senior Rider and dragon were caught in a spell that almost stopped their movement. While in this hex, Formora and Kialandí were able to cast wards around the pair, this time stopping their movement altogether. As soon as the wards were placed, however, Oromis and Glaedr lashed out with their minds.
A vicious mind-battle ensued, one that lasted for several hours. Oromis and Glaedr versus Formora, Kialandí, and the latter two's dragons, as well as the Eldunarí of the dragon Agaravel, whose body and Rider were killed by Kialandí and Formora. Glaedr, in a later recount of the incident, recalled the battle as distinctly unpleasant.
When it became apparent that the battle could not continue for much longer, Oromis cast a complex and extremely difficult spell that managed to free himself and Glaedr from their bonds. Just as Oromis finished his spell, Formora cast a spell that Galbatorix had learned from the shade Durza to bypass Oromis' wards and permanently prevent him from using magic. Formora's spell only worked in part because of Oromis' spell and he had the first of many seizures that would consequently plague him throughout the rest of his life. Glaedr fended off Kialandí, Formora, as well as their dragons in order to protect Oromis during his seizure and managed to separate himself and Oromis from them. During the struggle, Kialandí was impaled and badly wounded on the spikes of his own dragon and was therefore incapacitated as Oromis and Glaedr made their escape. Formora and Her Dragon, however, were able to follow.
In her bloodlust, Formora attempted to kill a weakened Oromis with a slice of her sword, but Glaedr twisted to avoid the blow and as a result, the dragon's left foreleg was severed. It was stated later by Glaedr that Formora's victory worked against her, for without the weight of his leg, he was able to escape. Eragon also felt an emotion from Glaedr that was described as a cold, pinching sensation, the one Glaedr felt when his leg was cut off, as if Formora's blade was made of ice, not brightsteel.
Soon after their failure with Oromis and Glaedr, Formora and a newly-healed Kialandí flew to the island of Vroengard, where with the other Forsworn, they laid siege to the Riders at Doru Araeba and destroyed them.
Some time after the Fall of the Riders, Formora died under unknown circumstances. It is possible, even likely, that she was driven insane as her Dragon Partner descended into primitiveness. It is also possible she was killed by Brom, or in an attack from the Varden.
Since Formora has not made a physical appearance in the books, the extent of her powers cannot be certain. It is likely that she was very skilled at swordplay, mental powers and magic, since she was a Dragon Rider and slew many others of the order. She also learned dark magic from Galbatorix and possibly drew power from some Eldunarí she captured during the fall of the riders. Paolini stated that Galbatorix eventually gathered most if not all of the Forsworn's Eldunarí for himself, so Formora power probably lessened since the Riders' Fall.