I picked up my Empyreal Tunic chestpiece off the Adventure dealer tonight as well as the upgrade power node. After applying the power node I see that the item can be upgraded again. Cool I thought! How many Empyrial Sourcestones will I need?
Well this second upgrade path shows that I will need a "Glowing Idol of Unholy Power" and this comes from the Archive of Flesh dungeon (?). It is not purchasable from the Adventure dealer.
I am disappointed with this shift in upgrade path, thought I'd post it here in case there were others that are not expecting this. Make sure you check the upgrade paths!