The Nidhwal (Plural: Nidhwalar) is a sea serpent that is related to the Dragons and Fanghur of Alagaesia. Native to Alagaesia's Western Sea, they were intelligent predators that could use the power of their minds to incapacitate their foes. The nidhwalar had heads that were long and angular and had tendrils that were about six feet in length coming off the crests above their eyes. Their jaws were large enough for a horse and rider to pass through unscathed and were filled with hundreds of glinting white, needle-like teeth. They also had giant oar shaped flippers coming off their chests. They also had a gemlike structure similar to a Dragon's eldunari in their chests that allowed them to remain at great depth for long periods of time. Despite this, they cared nothing for anything other than themselves, and their hunger was insatiable, to the point that they would readily eat their own kind.
By the time of Alagaesia's 8th century, they were rarely spoken of and had become extremely rare, though the most famous was a nidhwal that fought a Dragon named Fundor. Another dragon, Cuaroc, was a hunter of nidhwalar when his body was still alive. Saphira, Glaedr and Eragon encountered a nidhwal as they crossed the Western Sea to reach the island of Vroengard, when it attempted to eat Saphira. Glaedr in particular was surprised to see one so close to shore, as they usually were only seen several days' journey from land, indicating they had either grown bold or desperate for food since the Dragon Riders fell to Galbatorix.