Bonus sketch for this work: https://www.furaffinity.net/view/49725795/

So, this AU is more fantasy and fairy-tale-like (like the later Warriors books lol), so with a few caveats, turning into a monster here is not even a metaphor ;3

Such creatures are extremely rare and usually there is only one Rat King for three generations of cats. In essence, this is shapeshifter cat, who can take the form of a conglomerate of living rat bodies, paws and tentacle-like tails in a moment of anger, despair, or some other strong emotion. Usually the most powerful and dangerous representative of cat clans becomes the carrier of this curse, and it's impossible to remove it. A nice side effect... neither in normal form nor monster form, the Rat King can't be attacked by real rats)
And one more thing: non-believers and non-superstitious intelligent beings often can't see the form of a monster, continuing to perceive the carrier as an ordinary cat.