Puella Magi Madoka Magica meets My Little Pony in this crazy crossover! Spike is acting as a "Messenger of Magic," who grants the wishes of young ponies, but in exchange contracts their services as Magical Girls!
In front of Spike we find each Pony's Soul Gem. The Soul Gem is the source of a Magical Girls magical powers, though everytime the magical girl uses her magical powers the Soul Gem becomes "tainted". When "tainted" the Soul Gem loses it's light and becomes more and more darker. The color of the Soul Gem darkens also. The only way to rid the Soul Gem of it's "taint" is to use a Grief Seed. The Grief Seed will absorb the "taint" out of it. When a Magical Girl transforms, the Soul Gem transforms together with her and becomes a part of her costume. The Magical Girl can also transform the Soul Gem into a ring, which makes it easier for the Magical Girl to take it with her, and not losing it.
Which is your favorite Soul Gem?