Gruntan Kurdly was the fierce and very overweight warlord leader of the Brownrats. Like the rest of his tribe, he was painted in almost every color of the rainbow, and wore bone and leaf ornaments for decoration. For his size, he was quick - however, he was very lazy, and whenever he could do without walking he would ride in a litter carried by several members of his tribe. He had a great taste for hard boiled eggs; he also had many daughters, three of whom were kept constantly shelling eggs for him. Whenever he said he'd give someone a "bad case/attack/dose of the Kurdly's", it meant he planned on killing them.
Kurdly planned on laying a trap to capture the logboats of Luglug's Guosim tribe. However, as his scouts Noggo and Biklo had already been seen by the shrews, the ambush was a disaster. Kurdly himself nearly died by falling off a huge rock, and saved himself by grabbing onto a gorse bush. Only the (somewhat) professional help of Laggle, the healer, allowed all of the ensuing thorns to be removed from Kurdly's impressive bulk.
Enraged, Kurdly pursued the Guosim hotly and quite unsuccessfully, being tricked easily by the shrews' manoevres. Kurdly himself was badly hurt when Rangval the Rogue led the Brownrats towards Owch Mansions; he was stung on the tongue seven times by the Mansions' many wasp inhabitants. He was also attacked by a large non-poisonous snake, which Mad Maudie threw at him; the frightened snake coiled around his head, and beat him with the end of its tail.
The Guosim fled into Redwall Abbey. Gruntan resolved to take the Abbey, and set up a siege camp. When his scouts reported the presence of Vizka Longtooth's Sea Raiders and Gorath the Flame, Kurdly ordered his second-in-command Stringle to wipe them out, feeling cheated of "his" Abbey.
While Stringle chased the Sea Raiders, Kurdly's scouts reported the existence of a water meadow nearby. Kurdly was tempted especially by the presence of a swan's nest, as swan was the only egg Kurdly had never tasted. The Brownrats broke camp and moved towards the meadows. While passing Redwall, Gruntan spotted the Tabura on the wall, and mistook him for Gorath. Thus, he assumed that Stringle had lied to him when he reported that his band was fighting the badger.
Upon arrival at the meadows, a Brownrat messenger informed Gruntan that Stringle had surrounded Gorath and the Guosim shrews on a plateau, but needed help to finish them off. Taking this for another lie, Gruntan refused to help. He and Duggerlo later searched for the swan's nest. Gruntan tried to climb into the nest to retrieve the eggs he assumed were inside; however, he tipped the nest's entire contents, including some chicks and a fully grown female swan, onto himself. The swan, protecting her cygnets, viciously attacked and killed the Brownrat chieftain.