Groffgut is the secondary antagonist of High Rhulain.
Groffgut was the self-appointed leader of a gang of young water rats, who had aspirations to become a proper vermin band someday. He was leader purely because of his size and fierce temper, seeing as he had no real intelligence to speak of. He carried a rusted scythe blade with a rope handle.
While Groffgut's gang were camping in Mossflower Woods, near to Redwall Abbey, a rat named Frogeye captured the osprey Pandion Piketalon. Groffgut decided to eat the bird, but only after the gang tortured him with their spears. Luckily for Pandion, four young Abbeybeasts (Tiria Wildlough, Brinty, Girry and Tribsy) arrived on the scene, freeing Pandion and scattering the rats. Brinty beat Groffgut so badly that Tiria had to stop him; because of this, the water rat took an oath he would kill the young mouse as soon as he got the chance.
At first, the rats had no real plan of campaign, marching boldly up the path to the Abbey. Tiria spotted the gang from the Abbey's walltops; Groffgut threw a spear at her, but missed. She fired a slingstone at Hangpaw, killing him on the spot and raising the alarm; Groffgut's gang were forced to flee, with Skipper Banjon Wildlough and Brink Greyspoke hot on their tails. Groffgut eventually convinced his gang to jump into a bog to hide, which fooled the searchers but left all the vermin sticky and sludgy afterwards. After this fiasco, Groffgut decided to have revenge on the Abbey at large, instead of just Brinty.
The gang tried to sneak into the Abbey by standing on each other's shoulders to get a rope and grapnel over the wall. The attempt failed miserably, as Abbess Lycian happened to be taking tea on the walltop at the time and spotted them. She smashed the teapot over the pile of rats, causing them to topple on one another and flee into the night. Groffgut lost his tail in the scuffle, further boosting his desire for revenge on the Redwallers.
The gang decided to lie low in the ditch outside the Abbey for a while and watch for a new opportunity. It came when an injured gannet entered the Abbey grounds; Brinty shoved it out of the open gate, but his back was turned to the ditch in the melee. Groffgut saw his chance; he leaped out and cut down the mouse, thus fulfilling his revenge. He would have fled, but Skipper Banjon hurled his ancestor's lance and killed the rat, later throwing his body into the ditch for the hungry gannet to eat.
- Groffgut and Riggu Felis are the only villains known to have fulfilled a vow of revenge against a goodbeast.