Scripts for beginners.

Well, basically it's easy. We reserve a Necromancer hero and create a quest to "save" the main hero, making it technically empty (OBJECTIVE_KIND_MANUAL) so we can control it through scripts. In the scripts, we'll write something like this:

NowNecromancer = 0 --Condition that we'll set when the hero becomes a Necromancer

function prim_save_gg( heroName ) --You can name this function differently; it's just for clarity
      if ( heroName == Duncan ) and NowNecromancer ~= 1 then --Duncan is just an example. Replace it with the script name of your hero
           SetObjectiveState("scripted quest name", OBJECTIVE_FAILED) --Fail the quest to save the main hero
           sleep(10) --Give the player time to see the quest fail and realize they've lost the main hero
           Loose() --Fail the mission because our hero died before becoming a Necromancer
      elseif ( heroName == Duncan ) and NowNecromancer == 1 then --We lose the hero, but because we now have the condition that we're a Necromancer, we don't lose the game
           print("It's okay, the condition is met. He died, and that's life. Now I'm a Necromancer, maybe I'll go buy some skulls.")
      end
      if ( heroName == DuncanNecro ) and NowNecromancer == 1 then --Our hero is a Necromancer, and the condition that we made him a Necromancer has been met
            SetObjectiveState("scripted quest name", OBJECTIVE_FAILED) --Fail the quest to save the main hero
            sleep(10) --Give the player time to see the quest fail and realize they've lost the main hero
            Loose() --Fail the mission because our hero died after becoming a Necromancer
      end
end   

function TransformToNecromancer() --Here, we'll define what happens after the main hero becomes a Necromancer. How it's called is up to you.
--It's up to the author to decide what the conditions are.
      NowNecromancer = 1 --Set the condition so the game doesn't fail after deleting the main hero
      sleep(2) --Just in case, give the game time to process the information
      DeployReserveHero("DuncanNecro", x, y, GROUND/UNDERGROUND) --Place the new main hero so the game doesn't fail if the player only has one hero and no castle with a tavern
      sleep(2) --Also just in case, give the game time to process everything
      RemoveObject("Duncan") --Painlessly delete the main hero
end

Trigger(PLAYER_REMOVE_HERO_TRIGGER, PLAYER_1, "prim_save_gg") --Trigger to track when the player loses a hero (PLAYER_1)

Give it a try)