Blizzard has announced that you may start downloading files for the Diablo 3 expansion Reaper of Souls, as well as a "pre-expansion" patch that will be released in the coming months. The download will not affect your game at the moment, but will mean you'll have less to download when the expansion and pre-expansion patch are released.
"The background download data is encrypted and won't make any changes to the live game or Battle.net service until patch 2.0.1 and Reaper of Souls launch," Blizzard said in a post on battle.net. "In the meantime, you will be able play Diablo 3 as normal, and your in-game experience should remain unaffected by the download itself."
Blizzard said the download will be around 6GB in size and include things like models, textures, art files, and other upcoming content in patch 2.0.1 and Reaper of Souls that is already complete.
Reaper of Souls introduces a new character class, the Crusader, and a fifth act to the game. The expansion will also be a part of the Diablo 3: Ultimate Evil Edition when it launches on PlayStation 4, though Blizzard has yet to date this version.
The Reaper of Souls expansion will be released on March 25.
For further instruction on how to start the background download head over to battle.net.