Diablo II's Necromancer is returning to Diablo III in Rise of the Necromancer, a new pack that will come out next year. There's no word on price yet, but the pack will include the Necromancer as well as two character slots and some other cosmetic boosts.
In addition to unlocking the Necromancer as a playable character, players will also receive an in-game pet, two additional character slots, two additional stash tabs (on PC), a portrait frame, pennant, banner, and banner sigil.
Blizzard has more details in a Necromancer FAQ on their website, complete with exploding corpse gifs. The character pack will be available to anyone who owns Reaper of Souls on either PC or consoles.
Diablo III came out in May of 2012, with the first expansion, Reaper of Souls, following in March, 2014. Over the past four years the game has evolved quite a bit, overcoming its disastrous launch to turn into one of the best action-RPGs we've played.
It's interesting to see Blizzard release a solo character pack rather than a full expansion pack, and it perhaps signals that the company is looking at Diablo III more as a "service" game a la Overwatch than as a traditional retail product like their older games.