According to Norse mythology, Gjallarhorn (Old Norse "yelling horn" or "the loud sounding horn") is a horn associated with the god Heimdallr and the wise being Mímir. Gjallarhorn is attested in the Poetic Edda, compiled in the 13th century from earlier traditional material, and the Prose Edda, written in the 13th century by Snorri Sturluson.
Heimdall (Old Norse Heimdallr) is one of the Aesir gods and the ever-vigilant guardian of the gods’ stronghold, Asgard.
His dwelling is called Himinbjörg (“Sky Cliffs,” connoting a high place ideal for a fortress), which sits at the top of Bifrost, the rainbow bridge that leads to Asgard. He requires less sleep than a bird. His eyesight is so keen that he can see for hundreds of miles by day or by night, and his hearing is so acute that he can hear grass growing on the ground and wool growing on sheep. Here he watches and listens, holding at the ready the horn Gjallarhorn (“Resounding Horn”), which he sounds when intruders are approaching.
During Ragnarok, the gods know that their doom is at hand when they hear the dire call of Gjallarhorn signaling the imminent arrival of the giants, who cross the rainbow bridge to storm Asgard and kill the gods. The disloyal Loki, the particular nemesis of the unwaveringly dutiful Heimdall, is with them. Loki and Heimdall slay each other as the world burns and sinks into the sea.
The single mention of Gjallarhorn by name occurs in the Poetic Edda poem Völuspá, wherein a völva (a shamanic seeress in Norse paganism) foresees the events of Ragnarök and the role in which Heimdallr and Gjallarhorn will play at its onset; Heimdallr will raise his horn and blow loudly. Due to manuscript differences, translations of the stanza vary:
I know of the horn of Heimdall, hidden
Under the high-reaching holy tree;
On it there pours from Valfather's pledge
A mighty stream: would you know yet more?
Fast move the sons of Mim and fate
Is heard in the note of the Gjallarhorn;
Loud blows Heimdall, the horn is aloft,
In fear quake all who on Hel-roads are
