The Breath of Gjallarhorn was an event that was initially supposed to be done by Odin's son, Heimdall, but ended up being done by the Ghost of Sparta, Kratos. In addition, this marks the beginning of the apocalyptic war known as Ragnarök, thus bringing war to Asgard, which would ultimately lead to its destruction.
Norse Mythology[]
In Norse Mythology, during the onset of Ragnarök, Heimdall will herald the apocalyptic war by raising Gjallarhorn into the air before blowing deeply into it, which lets out a really loud noise, leading to the beginning of Ragnarök.
Beginning of Ragnarök[]
After Brok, one of the Huldra Brothers, was murdered in a fit of rage by Odin, the Vanir Goddess Freya was left infuriated and she became bent on destroying her ex-hubsand. Along with Kratos, after a while for the latter, she decided to change the group's priority from preventing Ragnarök to setting the very apocalyptic war into motion. Together with Freya's brother Freyr and Hildisvíni, they began making plans for their invasion of Asgard. Unfortunately, because Odin had learned of their plans while in disguise as Týr, they ended loosing the element of surprise. In addition, they have to enter from Týr's Temple, which proved to not be subtle. Furthermore, the Einherjar will hold the spaces in between in front of Asgard's Walls, which will become heavily garrisoned.
Knowing that they needed their own armies for the battle, each of them went to one of the nine realms to recruit the inhabitants living there; Kratos and Atreus ventured to Muspelheim to find Surtr whilst Freya went to Vanaheim to recruit Sigrun and the Valkyries that had been set free, and apparently alongside Byggvir and his wife Beyla. For Freyr's part, he traveled to the realm of Alfheim to rally the Elves while Hildisvini went to Helheim to negotiate with Hraesvelgr for permission to recruit the Army of the Dead, though he needed Mimir for it.
Sindri, who was left depressed and bitter by his brother's death, volunteered to travel to Svartalfheim to convince the dwarves to join in the invasion. When Kratos and Atreus reunite with Freya in Midgard, she revealed that the Vanir were ready to fight and that she gave the Valkyries (now called Shield Maidens) a new physical and corporeal forms, thus preventing them from getting corrupted again, unlike with Odin's dark magic. Moreover, the Light and Dark Elves had formed a truce and united as one, which would give the newly-formed alliance an advantage by air. The Army of the Dead, which Hildisvini had successfully recruited, became the coalition's vanguard on the ground for the invasion. When Atreus asked Freya if the dwarves will join in the fight, she tells him that she hadn't heard from Sindri yet.
The next day, after a full night of rest, Kratos, who agreed to become the general of the army, led his allies into the realm travel room of Týr's Temple, where he gave a short but harrowing speech about what was to come. As his allies renewed their resolve, Kratos blew into Gjallarhorn which sent out a colossal, earth-shattering horn-blast that echoed across the Nine Realms. At once, the nine Realm Travel doors in the temple ignited in blazing light, opening all at once as the World Tree shook from top to bottom. Seeing the Asgard gate slid open, Kratos and his allies surged forward into the realm of the Aesir. In their respective realms, the Elves, the Vanir and the Dead did the same.