Gjallar (or Gjallarhorn) is the Horn of Doomsday owned by Heimdall. It's prophesied that when Heimdall spots the Jotnar preparing to attack Midgard, he'll blow Gjallar, signifying the start of Ragnarök and alerting the other gods.
Magnus thought what would happen if someone accidentally blew the horn. Heimdall told him that he used the horn for taking selfies and that it was his personal phablet. Heimdall had accidentally headbutted the horn twice and all gods came running. He told Magnus that he informed the gods that it was a false alarm and made a .gif file of it.
- In the last book of The Chronicles of Narnia, The Last Battle, Father Time blows a horn at the end of Narnia calling all of the inhabitants, including the stars, to a door leading to Aslan's country. This is similar to when Heimdall blows Gjallar.