- "The Gjallerhorn is frozen over. Liberate it, and then use it to boost the spirits of our warriors!"
The Gjallerhorn was a horn used to rally the army of Asgard.
When the Frost Giants invaded Asgard, they froze the Gjallerhorn to prevent the Asgardians from marshalling their soldiers. Thor managed to defeat the Jotuns and release the Gjallerhorn. He then gave a sharp blast of the horn to call Asgard's warriors to arms.
The Frost Giants used this tactic again when Mangog opened portals allowing them and the Fire Demons to attack Asgard. Heimdall alerted Thor, who once again defeated them and rallied Asgard's forces.