Are you looking for Homura's reconstructed Soul Gem?
Spoiler Alert: This article/section contains details about future plotlines described in the Fanon series. Do not proceed unless you want to be spoiled. We warned you, seriously.
Dark Orbs (also known as Kajiuria System Deux) is a
counterfeited second version of the Kajiuria System constructed by Arstera, which they served as the Dark Magical Girl's main magic power source.
So far there are some Human females/Kajiurian clones who possesses the item:
Unlike Magic Gems, Dark Orbs had semi-unlimited magic. Some of them had the ability to turn into Witches in their own will. The system had one unique ability: their strength relies on their negative emotions.