The Lethrblaka, known as the Nrech to the Urgals, is the adult form of the Ra'zac. In the Ancient Language, their name means "Leather flapper." They could hunt anything in Alagaësia. Oromis stated in Eldest that it was strongly believed by the Riders that the Lethrblaka had followed the humans to Alagaësia. Only two of these creatures survived the Ra'zac War. The elves tried to eradicate the Ra'zac and Lethrblaka as quickly as possible. Two Lethrblaka escaped and laid eggs. The remaining Ra'zac and Lethrblaka (two of each) worked for King Galbatorix because he made a deal with them to work for him in exchange for their favorite food (humans). Oromis stated that they are cruel, twisted creatures, somewhat similar to dragons. However, they are described as being something akin to a twisted, cunning, evil dragon, as well as are probably not actually related to dragons.
The Ra'zac used the Lethrblaka (their parents) for reaching places that would otherwise be inaccessible, such as their lair in Helgrind, where the Lethrblaka lived with their offspring.
One of the only things useful against Lethrblaka is magic, thus only a Dragon Rider on his/her dragon would have a fair chance of killing them.
They had tight corded muscles, large, leathery wings, very strong hind legs, a thick hide, a beak "seven feet long" (according to the book), and eyes the size of a grown man's fist, with no pupil or iris. Its body had the appearance of a starved, furless dog and is roughly dragon-sized. From the description in Eldest, the Lethrblaka slightly resemble a Pterosaur, only with four legs. Their blood is blue-green.Their appearance has some similarities to the Ra'zac. They both have beaks, black eyes and they have no eyelids.
In Icelandic, Leðurblaka, which is pronounced almost the same, means "bat", which a Lethrblaka resembles.