The War of the Elves and Sauron
Elrond leads a massive cavalry charge, in what is presumably the full outbreak of war between elf-kind and Sauron. Sparked by the invasion of Eregion by Sauron after his deception of Celebrimbor was made known to Gil-Galad—and after Sauron had secured all but the three elven rings from Eregion—this war saw Sauron’s dominion claim all of Eriador, destroying the elves’ rule of the region and driving the survivors of Eregion’s fall westward.
Although Celebrimbor was slain during the assault on Ost-in-Edhil, Elrond’s host arriving in an attempt to defend the capital was saved by the intervention of Khazad-dûm’s forces, buying him and the survivor’s time to flee north: founding what would become known as Imladris, aka Rivendell. Given his prominence here, we’re presumably seeing Elrond racing to try and defend Eregion, rather than the first siege of Imladris that occurs later on in the war.
