Powerful Ancient Dark Beasts
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The Shadow Jormungandr and the Dark Mana-Absorbing Spiders originated from eggs, but such eggs weren't created out of thin air like Shadow Beasts and Shadow Soldiers were, they came from a mother, a mother that was the original queen of these two species created by Black.
Back then, in ancient times, Black required two powerful Divine Beasts that could become his companions. As a person that couldn't trust other people, the only creatures he could rely on were monsters he specifically trained and tamed to become his loyal beasts.
And he wouldn't make them out of just some random beast he found around, he had already decided that they would be special beasts made out his own powers, specifically modified to possess abilities and powers that could supplement him.