Horn of the god Heimdall in Norse mythology. In Old Norse it means "yelling horn" or "the loud sounding horn." It's mentioned in the 13th century Eddas.
Heimdall will sound his Gjallarhorn to awaken the gods to assemble before Ragnarok.
Gjallarhorn is referred to as the "Horn Resounding" in Terry Jones' 1989 Norse comedy Erik the Viking.