The shadow world spell can be unlocked on the Tyrants ability tree, and it's all the way at the end of one branch.
I chose shadow world in the skill tree, but couldn't find it in my spell book.
So, as far as I can tell, it's acquired like other spells..that is, there is a random chance that it will show up as one of the spells you can learn upon level-up.
I disagree with this approach for spells that are unlocked specifically via the skill tree. IMO, they should automatically be included in the sovereign's spell book as a leaned spell that can be used right away.
