Healing the necromancer can be done in various ways like the blood fountain or potions.
None of it works for healing the horde. Is there a way to heal it?
I never found a straightforward way, though there are items with vampirism and feeding traits that have healing effects on the horde. Also there are characters, like clerics, that also heal the horde.
Probably though the quickest is to go back to the crypt, banish them all and reload the horde.