The Second Battle of Svartalfheim was an event that took place during the Second Dark Elf Conflict. Part of a plan orchestrated by Thor to defeat Malekith, the battle took place on Svartalfheim.
In 2988 B.C., the Dark Elves attempted to wipe out the Nine Realms during the Convergence through use of the Aether. Bor, the then-king of the Asgardians, learned of the Dark Elves' intents and confronted them on their homeworld, Svartalfheim. A titanic battle ensued, during which the Aether was taken by the Asgardians and the Dark Elf leader, Malekith, fled with his lieutenant Algrim and various other survivors into suspended animation. They would then wait out the next 5,000 years until the Convergence would begin again and the Aether would reveal itself.
In 2013, Jane Foster accidentally stumbled upon the Aether after being sucked through a portal from the Convergence, and become its unwilling host. Foster unintentionally lured Malekith to Asgard, who infiltrated his lieutenant Algrim to transform into Kurse and allow a sacking of the realm by the Dark Elves in their search for the Aether.
Realising the Dark Elves' strength, Thor orchestrated a plan to lure them away from Asgard and into a confrontation on Svartalfheim, whereas he would be able to kill Malekith and destroy the Aether.
He did this by freeing his war criminal brother, Loki, who helped Thor in getting Foster away from Asgard and to Svartalfheim. Sensing this, Malekith and his ilk pursued them there.