The Skull Heart is an artifact that plays a major part in the Skullgirls story. Every seven years, it grants a wish to a woman. The wish can be of any magnitude, but it comes at a price if the woman's heart is impure: the wish will be twisted and she will become a Skullgirl. The Skullgirl is controlled by the will of the Skull Heart, though a woman of particularly strong heart and will can resists its influence to a certain extent.

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I'd say...make it powerful and mysterious. Although, these are the only lines, I will call back if the opportunity arrises.