More on Heimdall's Horn from Infoplease:

• Heimdall's Horn - Heimdall's Horn The sound of this horn went through all the world. Source: Dictionary of Phrase ...
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• Gjallar - Gjallar Heimdall's horn, which he blows to give the gods notice when any one is approaching the ...
• Giallar Horn - Giallar Horn (The). Heimdall's horn, which went out into all worlds whenever he chose to blow ...
• Dictionary of Phrase and Fable: H - Definitions, origins, and illustrative excerpts for words, phases, and literary allusions starting with "H"