

At an unknown point in time, Odin came into the possession of the horn, which he then gives to Heimdall in his aid in protecting the realm of Asgard when Ragnarök comes. What is known is that it lets out a very loud and powerful sound when blown. Not much is known about the origins of Gjallarhorn. The gods will awake and assemble together at the thing. After the enemies of the gods will gather at the plain Vígríðr, Heimdallr will stand and mightily blow into Gjallarhorn.

In chapter 51, High foretells the events of Ragnarök. In chapter 25 of Gylfaginning, High mentions to Gangleri that Heimdallr is the owner of the "trumpet" Gjallarhorn and that "its blast can be heard in all worlds". Using Gjallarhorn, Heimdallr drinks from the well and thus is himself wise. Just-As-High says that one of the three roots of Yggdrasil reaches to the well Mímisbrunnr, which belongs to Mímir, and contains much wisdom and intelligence. In chapter 15, the enthroned figure Just-As-High tells the disguised Gangleri about the cosmological tree Yggdrasil. In the Prose Edda, Gjallarhorn is mentioned three times, all mentions occurring in Gylfaginning.
