The Law of Cycles (円環の理 Enkan no Kotowari) is the rule that magical girls vanish when their Soul Gems turn completely black.
In addition to being a law governing the nature of magical girls, she is also a magical girl herself. The Cult of Hope reveres her as their Goddess. To her friends, she is known as Kaname Madoka (鹿目 まどか Kaname Madoka), and in a previous life, was the lead protagonist and title character of Puella Magi Madoka Magica.
Soon, however, it becomes clear that Madoka is still taking an active hand in the affairs of the new world, using visions granted via artifacts of her old life (such as the Ribbon and Clarisse van Rossum at full power) to give people advance warnings of preventable tragedies, among other things. Shizuki Ryouko, our protagonist, has met her at least thrice, in visions of her own.