Heimdall's Watch and the Bridge's Breaking

The god Heimdall is stationed at Bifrost's Asgard end specifically as its guardian, described in the Eddas as needing less sleep than a bird and able to see a hundred leagues by day or night and hear grass growing on the earth or wool growing on sheep, qualities that make him the ideal sentry for the one crossing point into the gods' realm. He carries the horn Gjallarhorn, kept ready to sound the alarm the instant any hostile approach nears the bridge, and this watch becomes decisive at Ragnarok, the prophesied end of the world, when Heimdall's horn call announces that the final battle has begun. The same prophecy holds that Bifrost itself does not survive the event: Muspell's sons, fire giants riding out under Surt's leadership, cross the bridge in the final assault, and the Eddic sources describe it breaking apart under them, unable to bear the weight and violence of that crossing even though it had held against every giant incursion before. The destruction of Bifrost is treated as one clear sign, among several, that the world order the bridge helped maintain, keeping the giants' realms separate from the gods' own, is ending along with everything else at Ragnarok.