Gjallerhorn

"The Gjallarhorn is frozen over. Liberate it, and then use it to boost the spirits of our warriors!"

- Heimdall

The Gjallarhorn was a horn used to rally the army of Asgard.

History
When the Frost Giants invaded Asgard, they froze the Gjallarhorn to prevent the Asgardians from marshalling their soldiers. Heimdall alerted Thor of this, who managed to defeat the Jotuns and release the Gjallarhorn. He then gave a sharp blast of the horn to call Asgards warriors to arms.

Trivia

 * In both the comic books and, the Gjallarhorn is blown to announce Ragnarok.