Not exactly selling the old Whistler here am I? I genuinely do reckon you'll find something here that you'll be able to use. For instance, the very nature of the monster and its habits suggest scenarios. Mostly Call of Cthulhu style investigative activity. So if you're stuck that way, etc, etc...
NOTE: I'm rather partial to Runequest 6, so I'll convert it to that sooner or later.
The Dream Whistler
Sometimes, the membrane between the realms of wakefulness and sleep grows threadbare. That untenable things of both beauty and madness sometimes find their way into the waking world, is beyond dispute. That those inhabitants of dream are violently forced to conform with the physical laws of our own bleak cosmos, is fuel enough for a thousand nightmares of horrors born afresh to a hunger, which must now feed.
One such horror is the Dream Whistler.
Arising from the chimera of amour which most often assails youth, in slumber they are amorphous clouds of half forgotten loves and desires. In flesh, they are cruelly rendered as obscene mollusc things as big as hippos (although no bigger than mice when first conceived, upon taking a lair, they quickly and unnaturally grow in size... a process which leaves them ravenously hungry). Their heads are always different: generally idealised representations of people the Whistler's original host once dreamt of. If that love was true and reserved for one person alone, then only one head will be present and near perfect. If that host longed for more than one person, then multiple faces will develop, with imperfections in their features growing with the number present. These slack jawed, drooling countenances are all sculpted from flesh, hair included, so as to resemble awful, glistening statuary. The creature has one functioning mouth set into it's upper trunk. Beneath this horrible lamprey like orifice are arrayed row upon row of pendulous mammary sacs.
Their means of locomotion are extremely limited, having to rely only upon long spindly arms and barely obedient cephalopodium. Ultimately they bind themselves to surfaces wherever they find themselves lairing, in a fashion akin grotesque barnacles. The lair is of particular significance to the Dream Whistler, being in a sense, a part of it's body (and hunting routine). They secrete themselves in lonely places of beauty...beneath crumbled bridges, in the dark of mossy forgotten crypts, at the bottom of crumbling wells, in the hollows of great dead oaks. Places where lovers might wander and carve their names into wood or stone and perhaps leave a little of themselves behind.
How do they get there? Some say that they are born from the death anguish of suicidal unrequited love. The monster manifests near a place where someone has taken their own life out of despair (this is partly true). Others suggest that (and these scholars are perhaps more accurate in their assessment), the Whistler gestates within dreams and whilst still tiny, manifest physically within the cerebellum, ultimately exiting the host via the nasal cavity. This can be the result of grotesque magics, but the spectre of spontaneous creation is also postulated as viable. If this be so, could our very dreams be merely incubators for monstrosities? If so, what then of our true natures?
The Dream Whistler, once created, secretes itself in the aforementioned lair and waits. A kind of hidden antennae set within the creatures' head area, picks up electromagnetic signals from nearby psyches. These are nearly always the minds of romantics, given to wandering and contemplation in places of serenity: poets, artists and lovers--- the forlorn, the lost, the pining and the mad. Once it has detected such a mind, it makes an imprint of the brains waveform and then 'pings' this waveform on a frequency only detectable in deep sleep.
The chosen, wherever he is, receives this waveform and it alters the very fabric of his dreaming. Any loved one that appear in the dreamers' sleeping worlds, can be replaced by an illusionary puppet under the control of the Dream Whistler. This puppet looks, speaks and acts exactly as the dreamer would expect of the loved one being replaced. It then attempts to lure the dreamer back to the place of serenity. This manifests beyond mere reverie. The suggestion or 'whistle' is strong enough to induce somnambulism. The poor slumbering victim will rise from his place of rest and make his way physically to the lair of the Dream Whistler. This may take several attempts over a number of nights and/or weeks. Sometimes the whistle is lost (lead blocks it as do more exotic, magical materials) and the victim awakens to find themselves miles from civilisation in the darkness of the countryside. In terror they flee home, but the process will only repeat itself, night after night.
Upon arrival at the lair of the Dream Whistler, the victim remains asleep, perceiving the image of the dreamt loved one waiting with open arms, drawing the sleep walker to fall into their loving embrace. In reality, the Dream Whistler's physical manifestation enjoys or needs to consume a certain amount of fatty tissue and compels it's victims to suckle at it's grotesque teats. These mammary glands produce something akin to an opiate/glucose/milk. The Dream Whistler will then nurture the victim for weeks. Throughout that time the poor dreamer never awakens, sucking greedily at the things narcotic breasts, insensible to the straining of their flesh and skin, the weird milk coursing through their bodies. In a short period of time, victims are flabby, oily mockeries of their former selves and ready to be plucked like gross fruit.
The ultimate fate of the Whistler's victims is to be messily devoured, lifted with ropey arms and stuffed into the waiting maw. This process is surprisingly swift, considering the measured pace of the Dream Whistler's hunting and fattening ritual; the victim is only barely aware of their own grisly demise as their somnolent bliss is ended.
Hunting the Dream Whistler is not impossible and upon discovery, it can be destroyed with flame and steel. The fact that it is largely stationary works against it. However, it is more than capable of defending itself and is able to use it's milk gorged victims as loyal and suicidal bodyguards (for their part, they perceive attackers as monsters invading their dreaming or threatening their beloved). The creature is dangerously psionic and is capable of employing surges of brain cooking energy waves. Finally, it's long, ropey arms clutch and grasp spasmodically and unpredictably, attempting to grab and then eat brave adventurers.
The final tragedy, is that upon the Dream Whistlers death, it's former victims are never able to truly forget their experience. They pine for that long dream wherein they were glutted upon ambrosia in the arms of their beloved. Following a period wallowing in a wasteland of cakes, pastries and other fattening foods (which they crave following the cessation of the Dream Whistler's milk), they are eventually found, having taken their own lives, at or near the old lair of the Dream Whistler. But before that moment comes, there is yet a worse horror: a freshly incarnated Dream Whistler (the same one?) crawls from the victims nasal cavity to nest anew... to whistle once more into the darkness for prey.
Armor 15 (blubbery almost-flesh), Move 0/80 (mature specimens moored in place/immature specimen)′, 7 Hit Dice, 42hp (an immature Dream Whistler has only 6hp), 2x grasping claws 1d6 damage each, Morale 10. If both claws hit, will lift and attempt to swallow. Save vs paralysation or end up in it's stomach where it's digestive juices deal d4 damage/turn. A swallowed victim can take no physical action. The stomach can digest one victim at a time.
It can forego attacking physically to unleash a wave of brain frying psychic energy. Everyone within 30 feet must save vs magic else suffer d10 damage. It's thralls are immune to this energy. Lead, mithril and adamant protected hats or helms reduce this damage to d3.
It's ability to initially imprint upon the dreams of it's victims requires another save vs magic to resist.
It's milk thralls have stats as normal humans, although their morale rises to 9 and they attack with such ferocity, that their unarmed attacks deal 1d4 damage (thrashing, biting)
Note: Obviously the Dream Whistler makes an eerie whistling sound...like a fragment of a tune. This whistling sounds quite disarming and out of place.