In Norse mythology, Gjallarhorn (Old Norse "yelling horn") is the horn with which the gatekeeper god, Heimdallr, announces Ragnarök. In Gylfaginning, Gjallarhorn is also the name of a drinking horn used by the god Mímir when he drinks from his namesake well with. In the latter case, the name Gjallarhorn may mean "horn of the river Gjöll".