Lost Song includes a completely original story which primarily involves the exploration of "Svart Alfheim", a new area of ALfheim Online comprised of a series of floating continents close to the World Tree.

How long is Sword Art Online: Lost Song?
When focusing on the main objectives, Sword Art Online: Lost Song is about 23 Hours in length. If you're a gamer that strives to see all aspects of the game, you are likely to spend around 46½ Hours to obtain 100% completion.