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

Yeah, at level 7 and with such low stats (what they will become if we still leave this presence).

Low stats are a natural consequence of powerful abilities; there's nothing wrong with that.

And then you can give one undead dragon this presence, and the other - incorporeality and HP.

In WGE, it was distributed exactly like that: the light necromancer dragon had incorporeality, and the dark one had terrible presence. However, they remained in the castle at level 7, and the damage of these creatures themselves is extremely low, so they mainly play with abilities.