Mehtan the Necromancer is an NPC (non-player character) that appears in Diablo III. It seems to me that some players are confusing Mehtan with other well known necromancers in Diablo lore (such as Zayl or Mendeln/Kalan).
Mehtan the Necromancer is a new character. You can find Mehtan the Necromancer in two different bounties/events in Diablo III.