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

I'm playing through the campaign as the Dwarves and encountered Astral Dragons – they are neutral there (they don't belong to the Necropolis). My question is, who was supposed to be there instead of them? I understand it should have been Death Knights, but it's impossible to defeat them with 450 HP and incorporeality when there are 40 of them. Also, they have an ability that removes the penalty for melee attacks – what's the point of that? Dragons can't shoot, so you could reduce their stats. And since they already have incorporeality, you could reduce their HP to 200 and remove the double strike ability. I haven't seen any Spectres yet, but I assume their HP is at least as high. I think that if they had a fatal blow ability but without incorporeality, 250-300 HP would be a good value.