From GameWatcher: “Blizzard is celebrating Diablo’s anniversary with the Darkening of Tristram event, which has returned to Diablo III.
Players will have noticed that some of Sanctuary’s cultists have started dropping clues, starting from a couple of days ago, that lead to a portal to Diablo’s past. That portal is now open and will remain accessible until January 31 when the Darkening of Tristram event is set to end.”
Source: N4G PC Diablo III's The Darkening Of Tristram Event Is Now Live