You come across him and his three skeleton minions, and he asks you to help him fight off some restless ghosts that the Cultists disturbed. This thread on Blizznet also discussed some of the known lore related to the Necromancer's apprentice. They do go on to confirm a special case where you run into the apprentice of the Necromancer from Diablo II during a chance encounter in Act II. They specifically state that all that is known are hints and vague references, deliberately. At this link you can see a video in which a Blizzard panel member from Blizzcon 2011 explains that generally, you don't know what happened to the heroes from Diablo II. So I've done some additional research, and I believe I have found the most credible Blizzard sourced information on the subject. They have no hope against a full onslaught from the Burning Hells. Sanctuary’s frightened inhabitants are turning to abandoned legends and prophecies for answers. Now, a star has fallen from the heavens-a grim omen that heralds the End of Days. I believes backs up the updated content in my first edit.

The official Diablo 3 Game Guide is now live, and includes an introduction which sets the stage for new players entering Diablo 3 without the background of the previous games.

Diablo 2 canon class characters