Considering the similarities a book of the dead isn’t a bad idea.
From what I imagine each faction to be I can see them all having access to some kind of regeneration ;zombies grow in number for each model they kill,the Restless will come back through the “Will” of their leader resurrecting them, the Necromancers would of course use magic which would be a double edged sword as it can be negated and the Vampires wouldn’t be able to resurrect the dead but there characters and other multiple LP units would be thorn in your opponents side as if left alone they could return to full strength.
I reckon the Restless and Zombies would be kind of “Basic armies” and unable to access other armies but they would have some units exclusive to their armies. Whereas the vampires and Necromancers could dip into zombies and restless but not access their best units.
