Gjallarhorn

The Gjallarhorn (Old Norse: Gjallarhorn, 'the resounding horn' or 'the yelling horn') is the great signal horn of Heimdall, watchman of the gods. It hangs at Heimdall's side as he stands his eternal vigil at Himinbjörg, the stronghold at the edge of Asgard where the Bifröst bridge meets the realm of the gods.

Heimdall requires less sleep than a bird and can hear grass growing in the fields and wool growing on sheep. He can see for hundreds of miles in any direction, even at night. With the Gjallarhorn beside him, he watches for any approach to Asgard, ready to sound the alarm.

The horn's sound carries across all nine worlds simultaneously. Its blast is so powerful that when it is blown in earnest, every being in every realm will hear it.

The Gjallarhorn is blown only once in earnest: at the onset of Ragnarök. When Heimdall raises the horn to his lips and sounds it across the nine worlds, it signals the beginning of the final battle. The gods wake, arm themselves, and ride out to meet their fates. The sound of the Gjallarhorn marks the end of one age and the beginning of what comes after.