It might be better if you choose one of the 2 races to work on.
What I've found about the necromorphs in dead space is that they are some sort of alien (no idea how they look) that go inside a corpse and then they and the corpse morph into some sort of freaky looking zombie (all different sorts)
if we know how the primary creature should look like we could easily create a send form (which would then be your pics)
do they use weapons or is it all melee?
Too me there isn't enough known to start creating a nation out of this atm.
What unit should there be?
how about commanders?
do you just buy each of the creatures or do you summon them?
basicly: tell me more

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