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Regarding the neutral creatures, I'm not sure how imbalanced they are. Death Knights might be weaker than Paladins or Spirits for highly leveled heroes; they are only imbalanced in small numbers (a 1:1 ratio is fine), and elementals might turn out to be weaker than standard level 4 units.
By green knights, I meant the ones from NCF, if they are possible.)
There is one special creature at levels 8-9; for some, it's the Dead Cyclops, for others, Wiglaf, for others, the Hell Spawn; for me, it's the latter.