The Darkening of Tristram is a special Anniversary event that becomes available in Diablo III every January. The first time the event was released was on December 31, 2017. It was the 20 year anniversary of Diablo I.
There are several Achievements that players can earn in The Darkening of Tristram event. If you don’t manage to complete everything all at once – don’t worry! The Achievements you earn one year “stick”, and you won’t have to re-earn them every year. You can pick up where you left off.