Starting in the Public Test Realm next week, Diablo 3 will receive a campaign that is based on the original game, which turns 20 years old on New Year's Eve. Like the original game, you will fight down the levels of a dungeon into Hell where you fight and kill Diablo. Pardon the spoilers.
The patch, called The Darkening of Tristram, seems to be taken light-heartedly by the company. They announce that it will add a low-quality rendering mode to pay homage to graphical limitations of 1996. More functionality, they also force the character to move in eight directions, which I'm not sure it's tongue-in-cheek or actually implemented for gameplay reasons.
Either way, you can check it out next week by joining the beta realm.