Item from A Song of Ice and Fire by George R.R. Martin
A legendary horn said to possess the power to bring down the Wall itself — attributed to the ancient wildling king Joramun, claimed by Mance Rayder, and feared by the Night's Watch as the ultimate doomsday weapon.
The Horn of Winter is the most terrifying 'what if' in Westeros — a weapon that could destroy the Wall and unleash everything behind it on the Seven Kingdoms. According to legend, Joramun blew the horn to wake giants from the earth, and the Free Folk believe it can shatter the Wall's magic and bring the ice crashing down. Mance Rayder told the Night's Watch he had found it, using it as leverage to demand passage south. Whether his horn was real, whether another horn exists, or whether the Wall's eventual breach by the Night King's dragon was the fulfillment of the horn's prophecy by other means — these questions remain unanswered. The horn transforms any scene it enters because it represents the possibility of total annihilation. It is Chekhov's gun at continental scale, and the question is not whether it will sound, but when and by whose hand.
Descriptions vary — Mance Rayder claimed to possess a large, ancient horn banded in bronze and etched with runes of the First Men. Whether the horn he displayed was genuine or a decoy remains uncertain. The true Horn of Winter, if it exists, is an artifact of the Age of Heroes, eight thousand years old.
Also known as: the Horn of Winter, Joramun's Horn, the horn that wakes the giants