You get Heroes by playing Nightfall and/or Eye of the North. You can't get them in factions or Prophecies.
I beleive your speaking of minions! Necromancers have skills that exploit corpes to raise minions to help with the battle. Necro's that use minions are called Minion Masters, or MM's.
edit: Awe man, Olim beat me. Damn these decrepid, arthritic fingers!