Currently on the PTR
As you may have heard, Blizzard is re-creating the original Diablo in Diablo III as a once a year event going forward. Each January you’ll be able to access the original dungeon with all four original bosses by way of an added questline called “The Darkening of Tristram” with bonus retro visuals, and you can test it out now on the PTR if you own a copy of Reaper of Souls.
All you have to do is log into the PTR version, select a character that’s cleared up to the “Old Ruins [of Tristram]” location, then head to that waypoint from the map. Walk towards the center near the fountain and you’ll find a blue portal that leads directly to the event. There’s a few quests strewn about the extra area, but it’s mostly a kill-fest.
If you haven’t already, you can also copy over all of your characters from the live version — just look for an icon in the top right hand corner of the main menu.