Those familiar with the game know that your base of operations is New Tristram, after the destruction of the originally named village after the events of the first game. The Darkening of Tristram is a short and fun add-on that lets players experience the quests and bosses of the first game, which was originally part of Blizzard's Diablo 30th anniversary party last year.
For those that have forgotten or have never experienced it, SVGA means 800x600 on a 14 inch monitor. And for online play over said dial-up modem (mine was 33.6k) you needed a full SIXTEEN MEGABYTES. So right now, I'm guessing you're either feeling pretty old or wondering what civilization must have been like on double digit MHz processors and dial-up internet service.
Well kids, respect your elders. Because all of us that lived through it when it was live thought it was simply glorious.
The gameplay stays identical to what you're used to in Diablo III, but quests like the Halls of the Blind, squaring off against Archbishop Lazarus, and standing toe to toe with the (original) Butcher will take you back to a simpler time. And let's not forget the original recipe highest level loot we all farmed and ground out for Lord knows how long - the Godly Plate of the Whale, King's Sword of Haste, and the Archangel's Staff of the Apocalypse - complete with classic Diablo icons. The same eerie music was there too, adding great ambiance while you fight your way to the Dark Lord. You can use any character you have to play through the event, but Blizzard's official recommendation is to start with a level 1 toon to get the full experience. I agree, because I went through it on my Torment IV barbarian and absolute facerolling was full effect.
At any rate, you have until the end of January to rack up the achievement rewards and a couple of transmogs, one of which may or may not be a very brutal red soulstone jammed right into your toon's dome.