Drizzt
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For example, right now, I want the Mage to acquire Omniscience, so there should be paths leading to it.
Actually, there's only one path to any ability. And to acquire Omniscience, you need to level up to at least level 29.
Got it. Here's a description of all the abilities: http://www.heroesofmightandmagic.com/heroes5/ranger_skills_abilities.shtml
Well... I think the Skillwheel is much more convenient.