Horn of Herrock
The Horn of Herrock is a black, twisted horn that is banded with gold. It is used to announce an arrival to a closed castle.
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 he arrives at Darry. It is sounded a third time when Jaime arrives at Riverrun .