Gjallarhorn

In chapter 15, the enthroned figure Just-As-High tells the disguised Gangleri about the cosmological tree Yggdrasil.

High mentions that Heimdallr is the owner of the "trumpet" (see footnote) Gjallarhorn and that "its blast can be heard in all worlds".

After the enemies of the gods will gather at the plain Vígríðr, Heimdallr will stand and mightily blow into Gjallarhorn.

[5] 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.

[6] A 9th or 10th century Gosforth Cross in Cumbria, England depicts a figure holding a horn and a sword standing defiantly before two open-mouthed beasts.

Bronze horn from 899-700 B.C. Påarp, Sweden.
Heimdallr blows into Gjallarhorn in an 1895 illustration by Lorenz Frølich
The Gosforth Cross panel often held to depict Heimdallr with Gjallarhorn
Detail of a copy of one of the two Golden Horns of Gallehus