(I couldn't sleep, was literally lying in bed thinking "this should be made of rats" so now I'm re-writing it so it is made of rats.)
A Ratmaster is a big floating ball, about the size of half a cow, of sleek, black, writhing rats. The rats flow like a dense school of fish. It has a face with two eyes made from radially symmetrical arrangements of still rat heads and gnashing mouth, the teeth of which are open rat jaws and sharp rat hands. Several stiff tail stalks stick up around the rim of the Ratmaster.
A Ratmaster is created when many very bad rats eat a n extremely magical book, or more than half of a wizard. They live out in the wilds because they are horrible and no-one normal wants to be around them. They might try moving into a village but no-one will deliver milk to them and they rarely complete paperwork so the situation rapidly becomes intolerable for all so instead it lives out in a vast cavern which it has carved with its rays, surrounded, cheese, scribe-apes and terrified zero-hours contract employees.
A Ratmaster is a very powerful creature, almost always it is the master of several kinds of lesser being who do its bidding, either willingly or out of fear of being turned into cheese. Usually its bidding is the stealing of things and threats of legal action. The only good thing about a Ratmaster being nearby is that there is unlikely to be a second Ratmaster as they hate each other. They fear and resent each other so much that if one catches sight of another they lock all of their eyes together in a terrified staring contest each one thinks that if they look away the other will gain the advantage and turn them to cheese (which is correct, they will). Ratmasters also think all other Ratmasters are horrifically ugly.
But what is the desire of the Ratmaster? Ultimate tyranny and watching of everything. The Ratmaster hates everything they do not own and everything they cannot see, so they like to own everything they can, gather it all in one place and assume a high, central position so they can see everything all the time, (assisted by their own eyes, floating 'Peepers', magic mirrors carried by servants and non-magic mirrors arranged specially).
The rats of the Ratmaster no longer birt normal rats, but 'Peepers'. These are small hairless floating rats with swollen hydrocephaic skulls. Their heads are full of light gas, their bodies shrivelled and small and their eyes large. They move by spinning their tails like propellers. The Peepers they float about, watching people, watching everything they do, even following them into the toilet to see if they are doing it properly.
The Ratmaster sends really presumptuous letters, though it can't write them itself. It usually ha s a scribe ape chained up with a quill and an endless roll of parchment. Most days the Ratmaster spends screaming in the voice of a hundered rats, dictating letters to people, often about very minor matters, like it saw them drinking milk right from the bottle before putting it back, not flushing the toilet or assaulting one of its Peepers. These letters are then sent to the offending party, nailed to local trees and pinned to the villiage notice board.
Since a Ratmaster doesn't have hands, and can't really do anything for itself all it can do is scare other creatures into doing what it wants. It thinks this is normal and doesn't really understand any other kind of relationship. It might think it wants friends or someone to spend time with but if any actual person hangs out with it the Ratmaster has very poor conversation, it can only really talk about the things it owns and how great it is, or things it wants to own and how terrible everyone else is. (You can sustain a conversation with a Ratmaster for quite a while by just agreeing with them and saying how terrible everyone else is). It is also extremely hypersensitive and utterly intolerant of criticism or correction of any kind.
The only way it can eat is by forcing someone to cook and serve it food. Then it has another servant tie a bib around its neck (which it doesn't really have), then it just throws itself face-first into a piled-up plate of food; GNA-NA-NA-NA-NA. Food goes everywhere but the Ratmaster is EXTREMLY upset if YOU do anything impolite. Really mortally offended. If, while it is chewing the glaze off a plate, you put your elbows on the table, or don't say please and thank you; SHOCKED I TELL YOU.
The Ratmaster terrible powers come from its Spells, which shoot out of the rat tails which stick from its ball like wild strands of hair.
Ape-Ray - Fires small monkeys. the monkeys pull your hair, get in your clothes, steal your shoes etc
Bad Ray - You must do wicked things each round until the effect lessens.
Bee-Ray - This fires thick, sticky honey which glues you in place and attracts ants.
Blind Ray - Makes you blind
Cheese Ray - self evident really.
Dance Ray - Forces you to dance everywhere.
Fold Ray - makes you a two dimensional paper person.
Glass-Ray - Turns you to living glass.
Gravity Ray - Makes your Up your Dawn and visa versa.
Long Ray - makes you long and thin, like a big long snake.
Love Ray - Makes you fall in love with what you see next.
Mad Ray - makes you think you are Napoleon, dictator of France
Mouse Ray - Turns you into a small mouse.
Pie-Ray - turns you to living Pie. (meat pies, not the American kind)
Pipe Ray - Turns you into a wind instrument. You can only speak when played by someone else.
Puff-Ray - Fires floating pufferfish like bubbles (they sting and you swell up).
Sad Ray - Makes you cry and be sad.
Sing Ray - Can't speak without singing.
Slow Ray - Makes you slow.
Thief Ray - Makes you steal whatever you can.
Wind Ray - (Is this even a ray really???). It fires blasts of hot stinking wind.
The Ratmaster usually constructs its lair by turning some caverns to cheese with its cheese ray and forcing its rats to eat it all. Fat, depressed rats and collapsed cheese prices are a sign that a Ratmaster is around.
The Ratmaster can vomit its own rats out and suck new ones up. It floats float over a writhing pool that looks like a shadow, but it moves. It is a flowing pile of big black rats with night-sleek fur who follow the Ratmasteraround and eat whatever it turns into cheese. Obviously, turning someone to living cheese and then vomiting rats is a bad end.