Nargott
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I agree that, considering only 5 skills are available for upgrading, dedicating a couple of skill points to less useful skills isn't ideal.
I like the idea of combining the less useful skills.
But I think that ideally, there should be only 2 of them.
A Light-Darkness skill and a Chaos-Summoning skill, or something along those lines.
Also, any grouping has its drawbacks, because in this case, the same types of units (Light+Darkness) and mages (Chaos+Summoning) end up in a less advantageous position; the Orc can counter their schools guaranteed, with one skill, not two.