When we think about the moon we must imagine the ways it can terrify us. A certain kind of horror can only take place under moonlight. Darkness is an honest monster. Moonlight lies. It transforms. It seduces. It is beautiful.
The moon is a white face pressing against the darkened glass of a childs bedroom in the night. The moon is that seeming in the word that suggests something else, bulging like a hand against the walls of a tent. The moon is many other things, good and beautiful, but after its distillation in the earth only the evil is left.
The stars and moon water the sunless earth with arrowheads of silver light. Most is reflected, some is absorbed, some is lost.
The lost light seeps into the ground like rainfall, but infinitely less. With different physics. Ignoring hydrological law. The Mondmilch descends in looping stuttering spirals like a coughing bird. The darkness slowly leaches away its lightness in the rock. It becomes heavy. Like white mercury. A single drop can take a thousand years to form. Beading invisibly in some forgotten crypt.
It forms pools of pure moonlight miles underground. An anti-patronus pool surrounded by the skeletons of mad dead artists. The walls are painted silver and the colours lost. Moonlight tends to blue, although the reason for this is unknown. The milk of the moon lives. And what would moonmilk want? Only the dark corrupted shadow of the wants of the moon itself. Art, transformation, mystery, metamorphosis.
The Mondmilch makes nightmare art from the echo of your own silent imaginings. The fears inferred by nightmares you recall. Imagine if your nightmare had a nightmare. A kind of anti-creation-equation that makes terror seem like a positive.
Mondmilch is motile and conscious. It moves like thick living mercury to surround you if it can. It kills you with monsters birthed from its pearly flows and the negative echo of your dread.
Take the WIS of the wisest PC. This is the HD value of the thing the Mondmilch makes to kill you. It can barely be described in words. Looking directly at it causes a save against paralysis every time as your mind fails to process the negative information that shapes it. It cannot be hurt by weapons of any kind and is immune to magic. But, it can be fought.
To fight the Mondmilch-beast the players must use their creative minds to reduce it to the level of mere nightmare.
If the wisest PC can describe their very deepest fear, what it is, and why they fear it, they can force the Mondmilch into that shape. Every further fear described can hive off half the remaining HD of the creature being fought. It will incarnate as the new nightmare made. After the first fear, all other PCs can participate in expressing their nightmares. Once per round each as a standard action.
This can continue every round, until the Mondmilch-thing is reduced to a crowd of individual One-HD horrors.
The nightmares will attack in whatever way seems most appropriate for each nightmare and each is vulnerable in ways you would expect from that particular creature, person or thing. PC's can fight each others nightmares.
Mondmilch lanterns are used by some for their imperishable glow, but they are dangerous. Heavy and bound in iron, they are fishtank prisons with small drips of the angry milk of the moon trying to escape inside.
I goes without saying that the light given off by Mondmilch has all of the magical and spiritual qualities of moonlight. Lycanthropy triggering, etc.