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I had drained all of the Sect Master's Mana and then pierced his chest with my claws, taking away his Mana Core and grasping it tightly!
I then replaced it with the Rank 3 Mana Core of a freaking Shadow Crow and then took away his Mana Core into my Inventory. After that, we healed him until he was fully back to life.
His Mana Veins ended destroyed and most of his internal organs were barely healed, but the flow of Mana was disrupted and now he was probably Mana-less, until he was to develop the ability to flow Mana into this new "prosthetic" mana core.
If he was still alive it meant it worked. Usually, those that get their Mana Cores extracted simply die, but we successfully replaced it with a low-grade Rank 3 Mana Core now! And it was also from a Shadow Crow, a Dark/Shadow Attribute Monster.