The Darkening of Tristram is a special Anniversary event that becomes available in Diablo III every January. The first time this event was released was on December 31, 2017. It was the 20 year anniversary of the release of Diablo I.
Players who compete the event with a brand new, Level 1 character, will earn the Butcher Pet. You must go through the Labyrinth, and slay The Dark Lord, in a solo game.