Horn of Winter
Ancient, magical horn in the A Song of Fire and Ice/Game of Thrones franchise by George R. R. Martin. It is also known as the Horn of Joramun or Joramun's horn.
When blown, it is said to have the power to bring down the Wall, an ice barrier three hundred miles long, about seven hundred feet tall, and wide enough for a dozen mounted knights to ride abreast.
It is mentioned in the 2000 novel A Storm of Swords