The Horn of Herrock is a black, twisted horn that is banded with gold. It is named after Herrock Kenning, the founder of House Kenning of Kayce.
Herrock the Whoreson used the horn as a signal for the whores of Kayce to open a postern gate, allowing his men to take the town from the ironborn for House Lannister.
A Feast for Crows
Ser Jaime Lannister orders Ser Kennos of Kayce to blow the Horn of Herrock to announce his arrival before the barred gates of Harrenhal, and does so again when they arrive at Darry. Kennos sounds it a third time upon their arrival at Riverrun.
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