Dracon Малласа
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It's better to take a mentor and level up secondary heroes (and give them education with mana attraction and dark revelation) and you will have a lot of dark energy... and 5% to necromancy will be just "a drop in the ocean." You can even ignore the Skill Academy.
In principle, everything is as you said, but I always want to level up a hero so that he has no equal in battle, and the Skill Academy helps with this.