Under the Magic tree there are 3 types of magic you can upgrade; Chaos, Order and Distortion. You need the appropriate level in a particular type of magic in order to upgrade a spell to that level. Say if you want to upgrade Fireball to level 2, you will need to have level 2 in Chaos magic first. If you mouse over the little upgrade arrow button by the spell, it will tell you what you need to do to upgrade.
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