Blizzard decided to celebrate the 20th Anniversary of Diablo by putting the original game into Diablo III. This was announced at BlizzCon 2016.
The Anniversary event is called The Darkening of Tristram. It will launch in January of 2017. The original Diablo game was released on December 31, 1996. Players who have the Reaper of Souls expansion will be able to enjoy The Darkening of Tristram event every January.