The Dark Elves deployed a fleet of Arks to fend off the Asgardian offensive in the First Battle of Svartalfheim in 2988 B.C. When the tide of the battle turned decisively in the Asgardians' favor, Malekith, the Dark Elven king, dropped the entire fleet onto both armies in a last ditch, spiteful maneuver.
Malelith, his lieutenant Algrim, and several dozen other warriors escaped Svartalfheim on the last Ark, entering a millenia-long period of hibernation. In their absence, Svartalfheim became a desolate wasteland marred with wreckage from the fallen Arks.
The Ark was used again thousands of years later when Malekith invaded Asgard to retrieve the Aether from Jane Foster. The attack failed and the Dark Elves fled, eventually chasing Foster to Svartalfheim, where Malekith successfully took the Aether from her.
Malekith traveled to Earth, where he tried to destroy the Nine Realms using the Aether. In a battle with the Asgardian prince Thor, Malekith was defeated and both he and his Ark were teleported to Svartalfheim by Erik Selvig's teleportation devices; where Malekith was crushed under the ship's bulk.
The Arks are massive in size and scope, capable of traveling at lightspeed between worlds. They are powered by black holes, which is a common power source in Dark Elf technology. They are equipped with cloaking devices powerful enough to mask themselves even from Heimdall, though not for much time. They also act as carriers, being able to hold several Dark Elf Harrow fighters.