Gjall

Gjall ("ringing horn"), is the horn carried by Heimdall, the watchman of the gods. It lies hidden beneath the third root of the World Tree Yggdrasil. With a soft tone Heimdall announced the arrival of the gods, but sounded it more forcefully in times of danger. At Ragnarok, the Twilight of the Gods, it will sound one final time and will be heard all over the world, calling the gods and the warriors to the battlefield.