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Phoenix is still heavily reliant on spells and vulnerable to darkness.
I don't know about the most useful ones, but here are some of the most interesting ones for me.
Summoning: Summon Hive I like both the appearance and the effect of this spell. Place hives and watch the enemy rest on the battlefield.
Darkness: Vampirism A very useful spell with many functions. Not only does it heal, but it also provides immunity to many different things.
Light: Teleport A useful spell with many applications.
Chaos: Stasis Field Not only is the visual effect cool, but it's also the only spell of this school that can be used for more than just dealing damage.