King Sarengo was a Pure Ferret, the father of King Agarnu and the grandfather of Prince Bladd and Princess Kurda. His personal servant was Mokug. Sarengo was a cruel beast, allowing his son to torture his slaves and beating Mokug with his studded pawring just for the fun of it.
One day, Sarengo and Agarnu sailed away from Riftgard on a plundering voyage. Originally, Sarengo had intended to raid Redwall Abbey with his Ratguards for treasure; however, he believed Brockhall would have more for the taking, so he entered the old badger's home first. He discovered Berussca the adder and her three babes Zassaliss, Harssacss, and Sesstra living there, and hastily retreated; he then scouted out a second entrance to Brockhall, writing down the directions to this back door for future reference.
Splitting his forces in half, Sarengo had Agarnu take one group of Ratguards through the back way while he again tried to force an entrance from the front. This proved to be a fatal mistake; Berussca met him head on, and knocked away his mace and chain, before he had a chance to attack. The cowardly Agarnu took the rest of the Ratguards and deserted, leaving Sarengo to his fate. Alone and unarmed, Sarengo engaged the mother snake in a death struggle, sinking his teeth into her spine and slaying her. Unfortunately, Berussca had already managed to strike him, and he died from her venom. As he was dying, Sarengo gave Mokug the directions to Brockhall's back door, and told him to guard them carefully. Berussca's eldest son Zassaliss later took Sarengo's crown for his own use.
It is interesting to note that, despite his death, Sarengo's discovery of the back door of Brockhall still resulted in the death of the adders as he had planned; many seasons later, a group of Redwallers followed the directions that had been given to Mokug and found the back door again, entering Brockhall and killing all of Berussca's now-grown children.
His body is described as a white monster by Ruggum, but he is not the White Ghost.