Let's see how well they expy'd it, though
From Black Destroyer: Coeurl
-Some kind of (probably black) cat. Tiger-ish.
-Is a hunter
-Eats id, a kind of phosphorous compound. Found in bone marrow. It smashes people up just to suck their bones dry. So a hunter which is uninterested in actual meat.
-Very long-lived, possibly immortal if kept fed. Can be killed though
-Has tentacles coming out of its shoulders (probably two) with either slender fingers at the ends or suction cups, depending on the version of the story you read. Can manipulate tools to an extent.
-Sensitive to/ has control over vibrations. Specifically radio waves, though other electromagnetic radiation is mentioned. This could maybe extend to sound? Unclear. Also there is a frequency of radio which is unaffected, so it seems limited in band. Interesting.
-Intelligent, with implied communication skills via radio telepathy (though no spoken language)
From MM 3.5: Displacer
-Catlike. (name-checks Bengal Tiger) MATCH
-Carnivore. SORTA MATCH
-Nothing special mentioned about lifetimes. DANG
(though describes them as horny) RACY LANGUAGE MATCH
-Displacement. A continual supernatural ability to bend light in order for their true location to be unknown i.e. some power over photons ...A UNIT OF ELECTROMAGNETISM. MATCH
-Speaks Common. WAIT YOU WERE DOING SO WELL
So some folks really focus on the photon thing, which is cool, having the displacer exist in a probability cloud, constantly hopping around. Like some quantum type stuff - Heidelburg etc. That's all really cool and I like that monster but they have already made that monster and I want to make a different thing.
let's do stuff:
-Keep the id thing, it's unique. Monsters don't discriminate which part of you they eat except for vampires.
-Also keep the near-immortality. Killable immortals are always cool.
-So an immortal who needs to eat bone marrow. Why? I'm gonna go with just a general gnawing hunger. Maybe it's related to fueling the displacing abilities, but I hate making rules for that stuff and it seems unfair to have it be a guess thing. I guess we'll come back to this
-Tentacles hell yeah. I like the finger ends, so it can pick up certain objects, just not use like guns or wands (do you need a thumb to use a wand? you do in hogwarts. maybe not for like druid style ones? there are some nature Darwinists who would probably be way into intelligent predators using wands)
-Displacement. Extending this to include other light manipulations would be ideal. Color shifting, chameleon style stuff, bending light for cloaking etc. Just in the immediate area. perfect for a stalker
-Other waves/radiation. The radio telepathy is cool, so if there was a pack they would coordinate (I doubt there would be a pack though. They live the solitary life of a shifting shadow)
ALSO: interact with spells that are blocked by lead sheet? They're basically xrays. So they could fool Detect Evil/Law/Kitties/whatever. So could play dumb for detect thoughts, or do a passable nystul's aura for detect magic. This would depend on it encountering that spell before, to get the frequency right and know how to scramble it. Same with Message - could be a good trap component (see below)
(also if you have any other radiation-dependent stuff you play with sitting around, go to town. like the aurora is extra strong around its lair; it lives in a thorium cavern; it can cook popcorn by concentrating on it; it reroutes migration patterns and alters compasses; whatever. If you give it free range of the spectrum the implications are pretty wacky, so i'm keeping it to radio, light and detect)
The fact that it is constantly misrepresenting its location is intriguing. Why? It has a deep fear of being pinned down, trapped, seen for what it is. This means constant deception, even to each other (so no packs, then) plus staying in open spaces, so no bottlenecks, tunnels, lead-lined rooms. Work through proxies. like Keyser Soze but a giant flickering panther. Hates being touched
- also it hates both full darkness and bright light - both are harder to manipulate to cloak itself. Grays and shadows.
- also since it fakes teleporting around all the time, it is insanely jealous of anything that can for real like blink dogs or wizards. Same with polymorphing or shapeshifting. like serious berserker rage level jealousy. This is generally the only way to ruin a coeurl's plan once it's in motion
-finds undead skeletons inconvenient because if the bones STAYED DEAD they would more easily give up their id. Like, just hold still for a sec Mr Bones this won't hurt
clearly if this thing was known about by civilization it would be thoroughly hunted since it is a goddamn bengal tiger that can turn invisible and fiddle around with magic. nobody would just let that exist in their forest. especially if there was a pack or den or something. staying secret and spread out is the only way to survive in this loud and probing world (oh yeah i forgot to mention they are still aliens. biowizards can't explain this GLORIOUS APEX BEAST. they stargated in or something)
what does it do though???for activities
some strategies it might employ for a steady source of bones:
-cloaked hunting obv. Pounce on animals from a hidden location. This would work best on nonsentient creatures
-visible lures, like a glamer of a little girl sitting in front of a tree, or a pile of treasure or whatever. basic illusions 101
-magic traps. like faking some magic items in a good pounce location; pretending to be a dumb/good ally; altering a magic message to tell someone to go into a trap. This is all adventurer-dependent so it kinda sucks as a real plan but still an effective tactic
-reversing detect thoughts waves to project thoughts to the user. a lucky coeurl with access to a helm of telepathy could purposely bequeath it to an unsuspecting victim then threaten, bully, plead, whatever
-same with radio waves. find somebody with the right dental work and convince them you are an angel or whatever. develop a cult of dentists who gladly find sacrifices for the dark prince fangwhisper. get fat
-generally the control of humanoids would be for the purpose of fomenting conflict. remain anonymous and unsuspected while coercing others to kill, then slurp clean the bodies.
Ideas to fight it (for the dullards)
-full darkness or bright bright light to throw off displacement
-hidden mundane trap. bait with bones
-find out its secret name and threaten to expose it. yes you can blackmail the giant space panther but good luck finding out. probably need commune or something powerful
-faerie fire still works. glitterdust has a 50% chance
(i just remembered something about it switching from chlorine to oxygen atmosphere without any problems so i'm sure there are implications from that. i don't really care though)
The coeurl is a big cat with grasping tentacles. It hungers for bones constantly, is intelligent, and probably very old, so can plan long-term. It can look different and mess with certain spells, and can send and receive radio waves. It sets traps and keeps to the shadows.