The Cat Miraculous is a ring that, whenever Plagg inhabits it, transforms the wearer into a black cat-themed superhero with the power of destruction (bad luck). The ring is currently owned by Adrien, who uses it to turn into Cat Noir.
The ring has a thick band with a circle surface surrounded by a lining with four pointed diamond shapes on each diagonal side. When Adrien isn't Cat Noir, the ring is silver and the circle is blank. But when Plagg enters the ring while Adrien transforms, it becomes black and a glowing green cat paw print appears in the circle. After Cat Noir starts running out of power, each toe of the paw print will disappear one at a time as an indication of how much power and time he has left until he turns back.
The Cat Miraculous grants its wielder incredible powers of superhuman speed, endurance, and strength. Its special weapon is a staff, which can be utilized as both a weapon and a communicator to other heroes. Its special superpower is Cataclysm, which gives the user the ability to damage or destroy an object, such as structures, akumas' hiding places, or metal bars. As long as the superpower isn't used, the wielder can remain transformed indefinitely. However, as soon as the superpower is used, the wielder's miraculous weakens and the transformation usually wears off in about five minutes.
The ring also has the ability to give the wielder the costume they truly desire.
If the Ladybug Miraculous and the Cat Miraculous are used together while also using a special invocation, the holder will achieve ultimate power. The person wielding them can make a wish that can alter reality. However, because the universe must maintain balance, every action has a reaction; for instance, a wish to become human would result in someone losing their humanity.
After Plagg consumes one of several special potions, or a food made with one, and the wielder transforms, they are able to unlock a power-up, an additional specific ability granted by each potion. So far, only seven potions have been shown, each with a unique color:
The green potion transforms Plagg into "Aqua-Plagg" and grants the wielder the ability to breathe underwater and improves their swimming capability with flippers on the feet.
The light blue potion transforms Plagg into "Plagg Glacier" and improves their ability to move on the ice with ice skates on their feet.
In the theme song, the Cat Miraculous resides in the raised center of the Miracle Box with the Ladybug Miraculous, both their sections creating a yin-yang symbol inside the box.
The Cat Miraculous is in the yang section (like in every yin-yang symbol there’s also some of each in the other). Some of the things that yang represents are heat, light, dominance, and logic.
Black cats are a symbol of bad luck, although they do symbolize good luck in some countries, such as ancient Egypt.
The ring can fall off while the user is transformed, which causes the user to revert back to normal.
In the original Ladybug PV, the ring has a very different design. It is entirely black with a cat face, the cat having green jewels as eyes and a red diamond-shaped jewel on its forehead.
Also, during the time the PV was produced and before the Miraculous theme was introduced, the ring would bring bad luck to its holder — even when he or she is not transformed. Additionally, the ring can't be removed unless a special requirement is fulfilled, as how Félix Agreste would be able to only if Ladybug kissed him.
In "Volpina", Cat Noir's legacy also dates all the way back to Ancient China.
According to the official Miraculous Ladybug Tumblr blog, there will be more holders of the Cat Miraculous revealed.
If the person possessing the Cat Miraculous used it for evil, it would be "even more than [extremely dangerous]."
For a staged battle for Mr. Damocles as the Owl in "The Dark Owl", Marinette creates a fake replica of the Cat Miraculous, which she later uses to trick Mr. Damocles' akumatized form and Hawk Moth.
In "Frightningale" Plagg thinks the bell on Adrien's costume is ridiculous, and he points out that such costume is what Adrien really wants deep down.
In "Style Queen", since Plagg mentioned having a Miraculous owner for 5000 years it's revealed that Cat Noir's legacy is over 5000 years old.