What horn announces the start of Ragnarok?

The horn that announces the start of Ragnarök is Gjallarhorn.

Gjallarhorn is the resounding horn associated primarily with Heimdallr, the vigilant god who watches over the gods’ realm and the bridge Bifröst. Its name is often translated as “hollering horn” or “loud-sounding horn.” Norse sources describe its blast as powerful enough to be heard throughout all worlds.

In the Prose Edda’s account of Ragnarök, the enemies of the gods gather at Vígríðr. Heimdallr then blows Gjallarhorn with all his might, awakening the gods and prompting them to assemble for the coming battle. The Poetic Edda also refers to Heimdallr raising the horn, though the surviving mythology is brief and sometimes enigmatic.

The horn belongs to Heimdallr, not Thor, and it is not a horn belonging to Yggdrasil or Valhalla. Gjallarhorn also has another mythic association: Heimdallr uses it as a drinking vessel at Mímir’s well in one passage of the Prose Edda. At Ragnarök’s end, Heimdallr and Loki are foretold to kill one another.