"Necropolis" - changes to creatures of levels 6-7.

Resurrection for the living??? Ha! NCF doesn't think so... (see Pontianak)

They hardly have any abilities. If you remove the special ability, then Knights on level 6 will also be affected. Then we'll have Knights without abilities, and Death Knights will practically be the same. And as someone correctly noted, "Death Knights without a special ability are not Death Knights" ;) But it's possible.

Then, let's say, give the Knights an attack with a curse, and the second one the one that Plague Zombies have.
Give Death Knights one attack with sorrow + an attack with blindness. The second one - an attack with fear + ? (maybe some kind of aura)
Give one an enhanced "hand of death" or something like that - instead of killing 1 unit, let it just deal damage; the second one - enhance the damage from the area-of-effect ability. Then we don't give them any spells at all, but give the Knights dispersion and blindness, for example.

Added 57 seconds ago
And mummies instead of zombies - just for good measure ;)