The Sacking of Asgard was a full-scale assault against Asgard by the Dark Elves during the Second Dark Elf Conflict.
In 2988 B.C., the Dark Elves attempted to destroy the Nine Realms by dispelling the Aether through the Convergence, but were thwarted by an Asgardian army led by Bor. Malekith, the leader of the Dark Elves, escaped the battle with his lieutenant Algrim and various other soldiers under his command and waited out the next 5,000 years in suspended animation.
Malekith and his kind were awakened when Jane Foster discovered the Aether after being warped to its location by the Convergence, and became its host. Taken to Asgard for treatment, Foster became a target for the Dark Elves. Malekith sent Algrim to Asgard under the guise of a Marauder, and instructed him to weaken the realm's defenses from within, allowing the Dark Elves to attack the realm with little resistance.