Gjallar

In 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.

Gjallarhorn is attested once by name in the Poetic Edda while it receives three mentions in the Prose Edda:

In the Prose Edda, Gjallarhorn is mentioned thrice, and all three mentions occur in Gylfaginning. In chapter 14, the enthroned figure Just-As-High tells the disguised Gangleri about the cosmological tree Yggdrasil. 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. Using Gjallarhorn, Heimdallr drinks from the well and thus is himself wise.

In chapter 25 of Gylfaginning, High tells Gangleri about Heimdallr. High mentions that Heimdallr is the owner of the "trumpet" (see footnote) Gjallarhorn and that "its blast can be heard in all worlds". In chapter 51, High foretells the events of Ragnarök. After the enemies of the gods will gather at the plain Vígríðr, Heimdallr will stand and mightily blow into Gjallarhorn. The gods will awake and assemble together at the thing.

A figure holding a large horn to his lips and clasping a sword on his hip appears on a stone cross from the Isle of Man. Some scholars have theorized that this figure is a depiction of Heimdallr with Gjallarhorn.

...
Wikipedia