What is the name of Heimdall's horn that will sound at Ragnarök?

Heimdall’s horn, whose blast signals Ragnarök, is called Gjallarhorn.

The name is usually translated as “the loud-sounding horn” or “hollering horn.” In the Prose Edda, Heimdallr owns the instrument, and its sound can be heard throughout the worlds. At Ragnarök, he stands watch over the gods’ realm and blows Gjallarhorn to warn that the enemies of the gods are gathering.

Gjallarhorn is mentioned by name once in the Poetic Edda and three times in Snorri Sturluson’s Prose Edda. The Prose Edda also connects the horn with Mímisbrunnr: Heimdallr uses it as a drinking vessel when he draws wisdom from the well belonging to Mímir.

The horn is often confused with other famous Norse objects, especially Odin’s spear Gungnir and Thor’s hammer Mjölnir. Gjallarhorn is not Odin’s horn, nor is it merely a modern nickname for Heimdall. Archaeologists have also linked horn-bearing figures on the Gosforth Cross, a 9th- or 10th-century monument in England, with interpretations of Heimdall and his warning horn, though that identification remains debated.