In Norse mythology, Heimdall possesses the warning horn Gjallarhorn.
Heimdall is the vigilant guardian of the gods and watches the rainbow bridge Bifröst. His extraordinary senses allow him to hear and see across great distances. When Ragnarok begins, he blows Gjallarhorn to alert the gods and the other forces of the final conflict.
The horn is not merely an ordinary battle instrument. Its blast signals that the long-foretold catastrophe has arrived. Heimdall then confronts Loki, and the two enemies kill each other in the final battle.
Gjallarhorn is sometimes confused with Gjallarbrú, a bridge associated with the realm of the dead. The names are related in sound but refer to different things. Odin’s horn is not Gjallarhorn; Heimdall is its traditional bearer.