Well, the way I would have it work is that you only get full access to the death spells if you are the Sidar. All other civilizations might get access to the SorceryI death spell ( if they get to sorcery), and that's it, no divine or level 2. They'd have full acess to earth and enchantment. Conversely, the Sidar get limited access to the earth sphere, and full access to death, and enchantment. The magic spheres would only be loosely tied together, as limited package deals.