Mura

Mura was a magical cat who was probably omnipotent, probably omniscient, probably omnipresent and probably also a genjutsu by Masuzu Urasabi's design. Nobody knows.

It served as the primary antagonist for Ren Houki's Magical Girls arc, though it at first tried to pass itself off as a helpful but decidedly cryptic spirit guide for him in the new world. The creature operated by something it called The Contract: it would offer its target, often handpicked, any wish of his or her choice in exchange for becoming a Magical Girl and working under it for the ostensibly altruistic goal of destroying witches. What it never told its targets, however, was that the wish would also put them into an alternate reality where every universal fact is rewritten according to the target's wish and contract (i.e, being a Magical Girl) and their newfound power also came at the cost of their soul.

Mura's true goal, assuming that it wasn't just a genjutsu by Masuzu, remain unknown, as it was destroyed by Fightin' Brawly in the aftermath of the Magical Girls arc before anyone could find out, but a part of his end-game (at least, in Ren's version of the Magical Girls arc) involved steady long-term plans to cause powerful kunoichi such as Hana of the Rinnegan to become witches so that it could gather more energy for its unknown purpose. It planned to do so by causing Ren to fall into despair and become a witch which would, in turn, hopefully have led to Hana doing the same, but was thwarted by its own target, who sacrificed herself in order to save Ren from becoming a witch. It later refocused its attention onto Lin Houki, who it deemed to be the next best thing, but was again thwarted by the combined efforts of Ren, Hamada Uchiha, Fightin' Brawly and Lin herself. Ren figured out that Mura was as equally obliged as its target to follow the terms of its contract, and with the help of the Hokage and his sensei, spun a wish for his sister to ask that would undo the reality that the creature created.