This is a pretty limiting definition, which is too bad. It makes it tougher to use those themes at low levels.
|not cosmic in any way, I just like this pic|
1. Weird Biology
Not just "it has tentacles and a bunch of gibbering mouths". Those things are weird because they differ sharply from normal terrestrial biology. They are highly unnatural (perhaps even impossible) without being hard to conceptualize. Impossible biology is the goal.
2. Weird Mechanics
Let the mechanics fit the theme. I don't want them to attack and defend the same way that regular monsters do. Their mechanics should reflect how unnatural they are. Ideally, the players would have a moment where they say "oh fuck, I just realized how this thing fights".
I'll admit that this one is debatable. I mean, you could write a good story about a warlock summoning some fucked-up abomination that merely drags itself around for a few pages before dying messily, poisoned by our atmosphere or something. But I want monster.
A really good follow-up post to this one would be Non-threatening Cosmic Monsters. Or at least, monsters that don't kill you. Can you write a monster that would horrify the players without threatening them? (Of course you can. That's why this is such a good prompt.)
On the wall of the dungeon, what appears to be a tiny tree grows sideways. It has no leaves, and its limbs are translucent as the thinnest skin. If it were standing on the ground, it would be about 3' tall.
All at once, the tree flies apart, and the separate branches come twisting through the air like stiffened bubbles. They hunt like a flock of fallen leaves, close to the floor. Some are wobbling forks, while others are linear, and coil through the air like arthritic worms.
HD 1 (HP 1) AC leather Bite 1d6
Move as butterfly Int 6 Mor -
Breed - On a hit, a dendricule creates a new dendricule with HP equal to the damage dealt. This ability works on any creature with a fleshy body.
If no other prey is available, dendricules will eat each other, eventually producing a mass of 1 HP dendricules. Then they will reform the "tree", and wait. They can wait a very long time.
It looks like a flat piece of ash, the size of your hand. Or perhaps a shred of black paper, something a magician could easily fold up and conceal between his fingers.
It flies through the trees like a hawk, and all at once it pivots along some invisible seam and heads towards the knights.
It corkscrews as it flies, spinning like a pennant in the wind.
HD 1 AC chain Attach -
Move as hummingbird Int 6 Mor -
Eyeball Attacker -- Regular armor is useless against a noctule. Instead, it attempts to burrow through the eye sockets. Fitted goggles give +2 Defense. Eyes clenched shut = +4 Defense. Eyes clenched shut with both hands covering sockets = +6 Defense.
Papery -- Any fire damage instantly kills it. If it would take any piercing/slashing damage, it instead splits into two smaller noctules, each one with half of the HP (round down). If this would result in a noctule with 0 HP, it instead dies.
Burrow -- After attaching to its target's face, it burrows in through the eye socket and consumes the brain through the optic nerve. This occurs automatically on the next turn, and in invariably fatal. The person's life can be saved if a torch is applied directly to the noctule (and the eye) during that turn.
Once a noctule has killed a target, they remain in the back of the eye socket for several hours, digesting the cognitive properties of their prey. During this time, they are iridescent, and fragmented memories can be seen on their skin. A careful hand can carefully remove a dormant noctule from the back of an eye socket and store them in a jar.
They reproduce asexually, by making nests in porous materials, often in a corner. These nests resemble inky stains.
When killed, all that is left of a noctule is a small amount of foul liquid.