One of the worst kept secrets over the past week is that Blizzard was going to announce Diablo 4 at BlizzCon, and after an apology from President J. Allen Brack about the Blitzchung incident, Diablo 4 opened the show with a new cinematic announcing it.
References to the three prime evils were throughout the video, which featured a group of four in a tomb seen in various areas of the Diablo series. They find a gate that can be opened by blood and after coercing their scholar to start opening it, and which evil begins to come out killing them and ominously speaking. Lilith is eventually freed, and is revealed to be the daughter of Mephisto, and is called the daughter of Hatred, and Mother of Sanctuary.
After that Blizzard explained that Diablo 4 will be darker, without clean victories, and full of blood and gore. They confirmed it will be an isometric Action RPG, and feature a classic class line up and focus on gameplay loop. The first three classes revealed were the Barbarian, the Sorceress, and the Druid.
Diablo 4 has been in development for a long time, and they announced that it has a playable demo at Blizzcon.