You should imagine that
Black Knights + Stone skin + Moro Dark companion + his shard he gives you means:
1 stack BK with wicked physical resist
always retaliates
with black helmet you can recruit even more of them
Eviln is best used on a corpse, then you move your BK stack onto it, rather than attack. They will get resurrected.
EDIT: You should also be aware that once you do Necromancer Meshmere's quest in Montero shafts, he will turn dwarves into Black Knights for you (as long as you refrain from killing him for a while, since he has a bounty on him).