|Book(s)||A Feast for Crows (mentioned)|
Ser Clarence's seat was called the Whispers, so named due to the disembodied heads that he would collect and bring home to his wife who would raise them from the dead. The dead heads would give him counsel and talk among themselves in a whisper. Ser Clarence collected the heads from a variety of people including pirates, lords, wizards, knights and even one king from Duskendale. Clarence is also said to have fought the squisher king.
A Feast for Crows
Brienne is taken into the captivity of the brotherhood without banners after fighting Rorge and Biter at the crossroads inn. The fevered woman dreams of fighting Clarence at the Whispers; the huge man's teeth are filed into points (like Biter), and he rides a shaggy aurochs. Brienne is without Oathkeeper in the dream, and Clarence strikes off her head.
Dick: Ser Clarence Crabb would have wiped his hairy arse with your Perfect Knight, m'lady. If they'd ever have met, there'd be one more bloody head sitting on the shelf at the Whispers, you ask me. 'I should have used the magic sword,' it'd be saying to all the other heads. 'I should have used the bloody sword.'
Brienne: Perhaps, but Ser Galladon was no fool. Against a foe eight feet tall mounted on an aurochs, he might well have unsheathed the Just Maid. He used her once to slay a dragon, they say.