Gjallarhorn ("ringing horn"), is the horn carried by Heimdall, the watchman of the gods. It lies hidden beneath the third root of the World Tree Yggdrasil. With a soft tone Heimdall announces the arrival or departure of the gods from Asgard, but he sounded it more forcefully in times of danger. At Ragnarok, the Doom of the Gods, it will sound one final time and will be heard through all the world, calling the gods and the warriors to the battlefield.

It is said that Mimir uses the horn to drink the waters from the Well of Wisdom.

Also known as: Giallarhorn, Gjall, Gjallar