Shukfir
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He probably chose her because he wanted to avoid a scenario where an overly intelligent and cunning offspring would usurp power.
Actually, he chose her because of the prophecy that the mother of the Messiah should be the last representative of an ancient lineage.
Plus, the choice was made either before or during Isabel's childhood. Agrail was assigned to watch over her from childhood. Isabel grew up right before his eyes.
There are no common resources like sulfur, mercury, gems, and crystals, because in this universe, according to the canon, they simply don't exist.
Actually, sulfur and mercury definitely exist in Ashan. They are present in Kaspar's quest in Might & Magic X as ingredients for curing Crag Hack's illness, which was inflicted by Sandro.
And crystals are generally one of the most valuable resources. They are formed from the blood of dragons and come in 7 different types. So they will also be in Heroes 7.