Brisingr is Eragon's Rider's sword. It was his third sword after Zar'roc, and his falchion, fourth if one counts the dwarf sword temporarily loaned to him. It was forged for Eragon by Rhunön, who used Eragon's body to circumvent the oath of "never" making another Dragon Rider's sword.
Brisingr was a hand-and-a-half sword with a curved cross-guard and a slightly longer hilt than usual; it was designed to be used for every situation Eragon might come across. It could fight equally well in a large battlefield as it could in a tight hallway. Eragon requested this from Rhunön because he needed "a sword for all occasions". Eragon would prefer to fight with one hand on his sword and the other on a shield, but circumstances did not always allow him to carry a shield. The sword was made so that he could fight with one hand as usual, but when needed he could grip with both hands for powerful blows.
The glyph for "fire" in the Ancient Language, "Brisingr", was engraved on both the blade and the scabbard. It had a dark blue scabbard, like the color of Saphira's back, with a leaf-shaped steel cap on the end of the scabbard. The blade itself was a lighter iridescent blue, like the color of Saphira's neck. The brightsteel contained cable-like patterns within it, and a flame-like pattern marked the transition between the soft spine of the blade and its hard edges. The guard was made from blued brightsteel. The blade was thin near the end in order to pierce through the upgraded armor during Eragon's time. The grip was made from black hardwood. Lastly, there was a large blue sapphire in the pommel held in place by four ribs, which looked like claws, made from blued brightsteel.
Whenever Eragon said "Brisingr," the sword burst into a blue flame. This unique property of his blade was foreshadowed by Christopher Paolini. Eragon's title among the Urgalgra was Firesword, originally referring to the red blade Zar'roc, and because of his magical ignition of his sword when slaying Durza.
When Eragon later showed his new sword to Oromis, Oromis started to say, "I wonder…" but did not complete the sentence, hinting that the swords true name might be Brisingr.
Christopher Paolini hinted that the name of Book 3, Brisingr, has more meaning than people are aware of. Eragon first used the ancient word "Brisingr" in the book Eragon when fighting the Urgals with Brom. At the time, he said he did not know the word or its meaning. Eragon also shouted out "Brisingr" when fighting Durza and when confronting the Ra'zac.
Brisingr was forged by the elf Rhunön by taking over Eragon's mind and using his body to make it. Because of a previous oath, Rhunön could not use her own hands to forge the blade. She used brightsteel, an extremely rare metal extracted from ore found in meteorites and fragments of shooting stars, which was traditionally used to forge Riders' blades. However, she could not find any after much effort and many spells of finding. Eragon and Saphira eventually found a nugget of the metal ore under one of the roots of the Menoa tree, as predicted in Solembum's prophecy. Rhunön (and Oromis) noted that it was the finest sword she had ever made.
Eragon wielded Brisingr during the Siege of Feinster. Whenever anyone inquired about the sword's name, Eragon was always interrupted. He used the sword to break through the wards of the city gates and portcullis. Although most people thought the name of the sword to be quite good, Angela thought of it as boring seeing that fire is hardly an eventful name. When Angela learned of the sword's name, she complained about Eragon's lack of imagination, suggesting the names Chrysanthemum Cleaver or Sheep Biter instead. Eragon immediately responded as to already having a Sheep Biter, referring to Saphira. Saphira however, was not too pleased at being called Sheep Biter.