Katastrophis

It has a lot of names. Sky Demon isn't even the most common one, it's just the most encompassing variation and tends to be the second favorite in most cases. I've also heard it called the Behengaud, Death Dragon, Ourogea, Abyssal Leviathan, just Leviathan, and so on and so forth. The most common name is Katastrophis. (Can be pronounced ka-TAS-tro-fis or KAT-as-TRAW-fis. No one really agrees about the emphasized syllable.)
It's an unusual entity in that there is only one, and yet many. Studies of this monster are the clearest signs we have of the existence of a multiverse, a fact that only exists to make my life more difficult as a scientist.
A/n: Yes, because Ruby's aggravation at having to potentially deal with multiple versions of herself or worlds where physics might work a bit differently is the absolute problem with this scenario.
Based on the nature of the creature, there can only be one in the universe. For another to exist would cancel itself out. They would destroy each other. And arguable a large chunk of the universe with them, if not all, hence why no efforts to drag in another creature from another universe and have them destroy each other.
There's only one known way to destroy it: the Amulet of Mirrors. Also the only way to contain it, either, at least until recently (see Battle of the Sky Demon for further details on other methods of containment).
Its powers are mainly physical, in how destructive it is and how utterly impervious to damage. However, it is also capable of draining life energy to strengthen itself, and while it doesn't need to do so to live, it does it anyway.
It is intelligent and capable of speech, even learning our languages in a matter of minutes (at least that's how it appears), but any attempts to talk it down have failed utterly.
One of my favored theories is that it only appears when summoned by someone, who is then able to exert a small amount of control over the entity.