Isenstar Lake was a large lake that bordered Gil'ead and Du Weldenvarden. Branching from it were the Ninor River and the Ramr River. The name was similar to "Isengard", from Lord of the Rings.
At some point roughly 60 years prior to the events of the Rider War, the Shade Durza was traveling near Gil'ead, where he happened upon a trio of boys named Haugin, Sharg Troutnose, and Nolf the Short along the edge of Isenstar Lake. The young boys, who had caught several fish including trout and sturgeon and had hauled them ashore, had been cruelly playing a game of watching the fish gasp for breath to see which would be last to die. Intrigued, Durza approached them with an evil smile and decided to let their game continue even after they had realized what they had done was wrong. All four watched until only the last fish, a sturgeon, remained. Durza took the sturgeon and then cast many spells over it, releasing it back into the lake. Durza then told the trio that the fish, being the strongest survivor, had earned its freedom. The Shade then said that for their cruelty, this fish, later named "Muckmaw", would haunt them to the end of their days, and then disappeared back into the forest from whence he'd come. Durza's foul spells allowed Muckmaw to grow to monstrous size and remain nigh untouchable for the next six decades, allowing it to attack anyone it came across as it traversed the lake.
During the Battle of Gil'ead in which Galbatorix killed Oromis and Glaedr through possession of Murtagh and Thorn, Islanzadi's forces drove the Empire's forces into the lake, killing many soldiers. It is implied that the Muckmaw then feasted on their remains, as their bones were heavily concentrated in his lair.
For the next six decades up until a year after the death of Galbatorix, Muckmaw reigned supreme in Isenstar lake, attacking boats, tearing through nets, devouring fishermen and werecats, and beating or drowning anyone he had no interest in eating. Muckmaw's armored hide and the wards Durza had placed on him made him impossible to kill for anyone save a Dragon Rider or an Elf, and the lake remained dangerous for anyone to fish. When Murtagh and Thorn arrived at Gil'ead, they were asked by the werecat, Carabel, to kill the sturgeon as part of a mission to save a werecat cub named Silna from the Dreamers, who had infiltrated the city's guards at Gil'ead. Muckmaw's death would impress the guards' captain, Wren, enough to invite him into their ranks.
Using a scale recovered from Glaedr and Oromis' grave, Murtagh tied it to a line of cord and cast the bait into an area of marshy wetlands full of rocks where the sturgeon preferred to lair. The huge fish took the bait, snapping the line despite Murtagh and Thorn's best efforts, with Murtagh even using the Name of Names to try and strip the fish's wards, which only kept returning. However, the fish did not flee and instead waited until Murtagh was within reach before dragging him into the water, where it attempted to drown him. Murtagh nearly drowned, but managed to kill the fish by breaking through his wards using a broken bone as a dagger, which he plunged into the sturgeon's underside, before Thorn hauled the fish ashore. Murtagh also recovered Glaedr's scale after cutting open Muckmaw's belly. With the monstrous fish dead, Isenstar lake was rendered safe once more, and the final piece of Durza's grip on the region was destroyed. Murtagh would let some nearby fishermen who had heard the commotion keep the sturgeon's body, but he took the massive head to present to Captain Wren.