RTF is a global modification that replaces all towns in the game.

As far as I know, Heroes 5 isn't entirely canonical compared to 6 and 7, as the Ashan universe was just being introduced in it. So, the neutral Death Knights have no reflection in the canon. It's a completely different matter that the Necropolis hero class, which focuses on strength, is also called Death Knights. Therefore, the conclusion is that Death Knights are just a type of necromancer.