The Eigengraü are terrifying, evil, obsessive stealth-predators that eat your eyes and plant their young in your flesh.
The Gegenschein are, noble, memoryless moth-men. Twilight warriors bearing bones wrapped in silent silk. They seek endlessly to challenge the deadliest foe and take its bones as trophy for their ritual sex.
They are one. The Gegenschein birth the Eigengraü and die, the Eigengraü plant the larval Gegenschein. The Gegenschein do not know this. The Eigengraü do.
A grey-black attack-caterpillar with neurotoxin beading on invisible spikes.
Predatory, malicious and effective. The Eigengraü will hunt you, consuming your fingers and extremities. Slowly. It will wait and take a finger each attack. Sometimes minute by minute. Sometimes day by day. You will not see its approach. You will not recall its attack. The nerve agent dewdropped on its radial filaments will steak its memory from your mind.
The Eigengraü is waiting at watching to see if you survive. It needs a host with a deep well of life brimming over within. The final power that breaks the chain and challenges the edge of death. The last attempt, the failure to submit. It needs born survivors to rape with insect eggs. It will take your fingers first to see how you react. If you do well it will take more. It needs a harmless host that cannot shake the egg but one with wits and strength enough to survive in the darkness. (It may also take soft tissue from your face, nose, ears, lips, subject to taste.)
If you show the capacity to survive its repeated invisible assaults then its final act will be to wait for you to weaken, and eat your eyes, one by one as your fingerless hands flail at its crawling flesh. Then, it drives its gonads into your stomach like a dagger strike. The creature dies. The young self-fertilise inside.
Combat with the Eigengraü usually lasts one round. It always wins initiative. If it hits you, or you hit it, the combat is over. The toxins take effect. You are allowed one swing after a successful hit by the beast before the chemicals wipe your mind. Once a combat with the Eigengraü is finished, no-one touched by the Eigengraü may speak of it. Not the player, not the character. They have no memory of it. They do not know what happened. They cannot find the Eigengraü.
PC's can exchange the HP damage for any Eigengraü attack into purely cosmetic damage doing on one HP each time. The price is the loss of a finger, thumb, or meaningful soft tissue from the face. When the character has nothing left to trade the Eigengraü will come for their eyes.
The Gegenschein are intelligent man-sized moths with samurai skills. Their grey wings glow. Like a cloud of cigarette smoke reflecting an unseen flame. The glow is only seen in natural dark, without the merest touch of artificial light.
Under lamplight the subtle greys are a fiendish camouflage.
Gegenschein have claws but wield improvisation like a blade. They make weapons from whatever they find. Rocks, bones, fire, light, patience and rope have all been used.
They grow from eggs maturing in a hero's guts which kill the host. The Gegenscheins first act is to consume the corpse that makes its crib.
The Gegenschein has almost no long-term memory. It is driven by instinct to do good. Its final mating-flight requires a dowry of bones. Only those Gegenschein who have repeatedly sought out and fought the most dangerous and evil creatures and survived will be allowed to mate and breed. They make a trophy of a single bone and wrap it in silent grey silk. These bones are read by prospective mates. They will assess the progression and challenge of each kill. If a Gegenschein does not show constant advancement, courage and skill, it will be denied.
As soon as its eggs are fertilised and hidden, the Gegenschein dies. It never knows what it has created.
The names and places it has seen are written on its wings in silvery thread. It wrote them there. It has no other way to recall where it has been and who it has met. This makes old moths more visible than the young, and more dangerous. Having survived so long and faced so much, they will be difficult to defeat. The last sight of many monsters is an odyssey of names in silver on invisible wings as the Gegenschein swoops down.
If hero's are powerful enough it will challenge them, but tracking is more likely. PC's summon badness from the earth. The Gegenschein will wait for something truly terrible to attack, then dive to engage it alone.
They do this quite a lot and have gained a reputation for heroism as a result. A kind of angel to the weak and lost.