The Dark Elf conflict is a conflict that lasted for thousands of years. Dating back to 2988 B.C., when Malekith, the insane, genocidal leader of the Dark Elves, attempted to use the Infinity Stone known as the Aether to eradicate the Nine Realms. After the the Dark Elves were defeated by Asgardians led by the then-king Bor, Malekith and several others of his species went into hibernation.
In 2013, during the Convergence, Jane Foster located the Aether and unintentionally attracted the attention of Malekith. A series of confrontations followed, spanning across all the Nine Realms, ending with Thor defeating Malekith at the Battle of Greenwich.
First Dark Elf Conflict
In 2988 B.C., the leader of the Dark Elves, Malekith, planned to make all of the Nine Realms pitch-black using the power of the Aether. Soon the Asgardians heard of Malekith's plan, so to protect the Nine Realms, an army of Asgardians led by their king, Bor, went to stop the Dark Elves in the First Battle of Svartalfheim.
Soon the Asgardians managed to take the Aether from Malekith. When the Asgardians began to get the upper hand in the battle, Malekith sacrificed everyone on the battlefield, even his own men. The remaining Dark Elves went into hiding and hibernated until the next Convergence.
Hunt for Jane Foster
Nearly five-thousand years later, the Convergence once again took place. Jane Foster, a human astrophysicist, came across a portal while studying the event, and accidentally warped herself to the Aether's hiding place. Jane unintentionally awakened the Aether, which possessed her body, using her as a host.
Alerted by the Aether's activity, Malekith and his brethren immediately woke from their ancient slumber and followed the Aether's signal to Asgard, where Jane had been brought by Thor in an attempt to remove the Aether from her body.