The Gjallarhorn (Old Norse “Resounding Horn”) is one of the most prized treasures of the gods. Heimdall, the gods’ ever-vigilant sentry, keeps watch over their celestial stronghold Asgard day and night with the Gjallarhorn in his hand. During Ragnarok, the final battle in which the cosmos will be destroyed, the giants will march upon Asgard. When Heimdall spots them heading for the gods’ sanctuary, he will let out such a huge blast from the Gjallarhorn that it will be heard throughout all the cosmos. Then all of the gods – and, indeed, all living beings – will know that their doom has arrived.
Unfortunately, the Gjallarhorn’s appearance is never described in the sources, so we’re left to guess. However, it seems safe to assume that, being one of the gods’ most cherished possessions, it would have been imagined to be more exquisitely wrought and carved than the finest horn used among humans.
Horns were one of the oldest Germanic musical instruments, and are well-attested in the archaeological record. Some of these horns seem to have possessed religious significance – might they have been the earthly counterparts of the mythical Gjallarhorn?
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 Davidson, H.R. Ellis. 1964. Gods and Myths of Northern Europe. p. 29.
 Simek, Rudolf. 1993. Dictionary of Northern Mythology. Translated by Angela Hall. p. 111.