"Two decades ago, my mentor faced the forces of evil and emerged victorious. I cannot hope to surpass his achievements, but I will try to bring balance to the raging forces that threaten to tear the world asunder in these days. The spirits are restless near Caldeum, I must quiet them."
Mentor and apprentice is a common concept of all novels around the world. And it is a cliche concept that apprentice will achieve what the mentor truly wanted to achieve.
So, in act II Mehtan the apprentice of unknown necromancer, gives you a quest to help him free restless spirits. Maybe he is searching for a specific spirit? perhaps his mentor?
What if Witch Doctor the only class that can walk the spirit world could help Mehtan find who is truly looking for. He could also find Deckard Cain in the spirit world and tell Mehtan to summon Deckard's spirit back to life. Or the leader of Horadrim Jered Cain.
When Mehtan successfully learn the true form of necromancy, he passes on to his future apprentices: a new hero class slightly different from the original necromancer but mixed with something else(I dont know..) Mehtan becomes the new legacy of Necromancer of the Caldeum and restores the city together with Asheara and Hakan II