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Can't be done.
Second option:
Mine specialist.
When capturing mines, the hero immediately receives a week's worth of mine resources.
Third option:
Friendship with death.
When less than 10% of creatures remain on the battlefield, they gain the "ethereal" ability.
(or receive a free spell, independent of the hero's skill in the Light magic school)
And one more question:
Did Iris (the griffin queen for Haven) specifically receive Dark magic at the beginning? Wasn't she supposed to have it?