Slagar the Cruel
He was originally one of Cluny's henchmen, known as Chickenhound, but later became Slagar, with a mask to hide his face, as some of his skin was torn off as a result of an attack by the snake, Asmodeus (though it has also been said that he did that to himself).
With several rats and weasels, including the spy, Vitch, Slagar, posing as a magician, put everyone in Redwall Abbey to sleep and took all the kids prisoner, including Matthias's son Mattimeo, to sell them as slaves to the evil Malkariss. Slagar twisted the story of how he became Slagar the Slaver, telling Mattimeo that Matthias was the one who injured his face. Though he attempted to lead a false trail and prevent Matthias from following him in any possible way he could, Matthias and many others with him were able to stay on his tail. At one point he threw his henchman Fleaback (Wedgeback in the book) over a cliff when he failed to prevent the prisoners' escape (though Slagar caught them again shortly after).
Eventually Slagar reached Lomehedge, turning his minions against one another while taking Vitch and Nadaz with him, and gave the prisoners to Malkariss, selling Vitch to him as a slave as well. Malkariss eventually agreed to give Slagar the land he promised and an army of rats, threatening to kill him and his guards if he betrayed him. After Malkariss was stoned to death by his slaves, Slagar attacked Matthias, but when the slaves attacked Slagar, he ran, sneaking to the surface by climbing up an old abandoned well. When Vitch attempted to escape, Slagar killed him with his bolas. Matthias and Orlando the Axe ran at him to kill him. When cornered at the edge of the empty well, Slagar threw a boulder at Matthias and Orlando, but they swung their weapons and destroyed it, sending the rocks flying at Slagar, causing him to lose his footing and fall down the well to his death.