I bloody love this faction. The way you describe how it plays is so evocative and really makes me want to play it for real.
I love the term 'The Rotting Necropolis'. I love the idea of The Rotway and having to create a chain of infection and if it's cut off from the source, it dies. The theming of this whole faction just seems fantastic to me.
I am curious why you need 50 Spore cards if you're only shuffling in 10. If I understand correctly, you're spending Spore cards in your hand to take Call of the Necropolis actions, placing them into your Spore Deck. Then in Evening, you're taking between 1 and 4 of them from the Spore Deck back into your hand. So it seems quite cyclical and 50 seems like a huge amount. After shuffling 10 into the regular deck during setup, do the other 40 all go in the Spore Deck?
Anyway, outside of the usual balancing concerns that can only come from an insane amount of testing, my first impressions are that this faction is fantastic and my favourite one I've looked at so far. Great job!