When the Watch arrive at Craster's Keep seeking food and shelter, Karl is openly antagonistic towards Craster, accusing him of giving his pigs better food than them. Along with Rast, he shows an interest in Craster's daughter-wives.
Following the death and funeral of Bannen, as tensions between Craster and the brothers of the Watch run higher, Karl insults Craster - calling him "a daughter fucking wildling bastard" - after the wildling demands the Watch to leave his hall and threatens to kill anyone who insults him. Craster charges against Karl but the black brother then kills him, stabbing him in the throat with a dirk. Karl's action triggers a mutiny that causes the death of Lord Commander Mormont.
In the A Song of Ice and Fire novels, Karl, called Clubfoot Karl, is a steward of the Night's Watch. Along with Chett, Lark, Paul, Dirk, Ollo Lophand and others, he had conspired to kill Lord Commander Mormont, Sam and other individuals and desert the Watch, but their plan was interrupted by the sudden snowfall and the subsequent White Walkers' attack. He is among the traitors who committed the Mutiny at Craster's Keep. However, Dirk and not Karl is the one who kills Craster and takes one of his wives as hostage.