Daggers were weapons that resembled swords, only shorter and lighter. In Eldest, Roran rebukes one of the men from Carvahall for wasting time with dagger tossing. Despite the boy claiming he had seen a man hit a bird eight out of ten times with a dagger, Roran points out that it only takes one missed throw to cost him his life.
The Werecat Solembum often wielded a dagger when in human form. In Eragon, the Ra'zac used daggers: the shorter one tried to kill Brom with one and the taller one picked one off the ground and threw it at Eragon. Brom jumped in front of it and received the wound that would take his life. In Brisingr, Dûrgrimst Az Sweldn rak Anhûin assassins used daggers in an attempt to kill Eragon.
Applications in Combat
Daggers were not often used in battle due to their short reach and inability to penetrate thick armor. Daggers were favored by assassins, however, because they were small and easy to conceal. Daggers could also be thrown accurately, which swords could not.