The Darkening of Tristram is a yearly event that remastered the original Diablo game in the Diablo III engine.
Blizzard Entertainment announced that the Darkening of Tristram is returning in January 3, 2021 at 4pm.
Stop the cultists in the Tristram Cathedral and enter the portal to start the event. Among the rewards you might find in the event is a Butcher pet, two Portrait frames, a battle standard, and other surprises.
The cultists will begin their ritual on December 31 at 4pm. (Thanks, Corey)