What is the name of Heimdall's horn that signals Ragnarök?

The story behind the answer

Gjallarhorn is correct because it is the resounding horn of Heimdall, the ever-watchful guardian of the gods. When blown, its blast is heard across all Nine Realms, announcing Ragnarök—the final, world-shaking battle of Norse myth. None of the other options are names from Norse sources.

Context tip: Heimdall stands watch at the rainbow bridge Bifröst, with senses so sharp he can hear wool grow on sheep. “Gjallar” means “yelling/resounding,” so think “the yeller horn” to remember it signals doom. In some traditions it’s also depicted as a drinking horn, adding to its iconic status. When you picture Ragnarök beginning, imagine Heimdall lifting