You’ll need Reaper of Souls to buy and play the new pack
The pre-BlizzCon leak spoiled the surprise, but heck, there’s no raining on this parade. Folks have demanded the Necromancer class for Diablo III and that’s exactly what we’re getting next year. He (and she!) will make it into the game in the “Rise of the Necromancer” pack, which will require owning both Diablo III and Reaper of Souls to access on PC, and the Ultimate Evil Edition on Xbox One or PlayStation 4.
Blizzard isn’t talking pricing yet, but the pack will also include “an in-game pet, two additional character slots, two additional stash tabs (on PC), a portrait frame, pennant, banner, and banner sigil.”
This Necromancer is a “reimagining” of the beloved Diablo II class, and the designers say they’ve created some distance between it and the existing Witch Doctor class in terms of “theme and play style.”
Restoring the Balance—Necromancer Overview [Blizzard]