It is said that the quickest way to a man's heart is through his stomach. In Orrorsh, the quickest way to a man's soul is through his health. For there is no fear greater than the fear of death, and when faced with that fear, a man will do anything to avoid it. Anything.
The Gro'n Bones (pronounced halfway between "Growing" and "Groan") is a member of a large class of Horrors all centered around the promise of a cure for a fatal illness uncureable by normal means. On Gaea it is usually Tuberculosis or Smallpox, but on Core Earth TB no longer raises anyone's hackles, if they can even recall what it is, and Smallpox is thought long banished. But no one can help but shudder at the mere mention of "Cancer". To capitalize on this, the Gaunt Man and his attendant Nightmares (primarily Ahjebax, though Skutharka's modern ways and Dr. Sconce's medical knowledge have also led them to explore "cancer") have designed a number of Horrors to specifically play to that fear. The Bones are a Horror based around Leukemia, cancer of the bones. Usually, a Nightmare or one of their servants will approach an individual with leukemia, play up their fears not only of the disease, but also the cure, which requires lengthy surgery to replace the bone marrow, in addition to numerous chemotherapy and radiation treatments. But in the end the Gaunt Man has found that he only has to offer one simple thing: an easy way out. Faced with the propect of death, or even a few months of agony, all too many will take the easy way, no matter what the cost. Of course, as with most roads to Corruption, there is no cost, to start. But in an ironic twist that so often follows in the wake of the Occult, the cells that had threatened to eat away at the person's body have merely been replaced by a larval Horror that will eat their very soul.
In it's intial stages, the larval Bones does eliminate the cancer. No trace of it will be found by medical means (and the Bones will use it's Corruption rating to hide from other forms of detection). In fact, the "host" will find themselves feeling even better than when they were diagnosed, better than they have ever felt before in their life. Old wounds will begin to heal, starting with broken bones (where the Bones has, naturally, taken up physical residence), curing any blood disorders, from anemia to infection, and even healing old scars and regrowing lost hair. (Lost limbs or organs are usually not regrown, as the Horror is limited to improving what it already has.) It during this time that those around the host will notice a change. (Though to be sure, they were dead the instant they agreed to allow the Occult to save their life.) They become more surly and quick to anger, often jumping to the worst possible conclusions ("I feel it in my Bones!"). They begin to shun daylight, prefering to stay inside, and better yet, below ground. Left to their own devices, one day the Bones merely disappears, joining the legions of Horrors at the Gaunt Man's disposal, leaving behind an intact bag of flesh. Usually where it will be found by those frightened by such things.
(Of course, Adventurers in general and Monster Hunters in specific are rarely complacent enough to leave well enough alone. If confronted before maturation, the host is more than willing to escalate any attempts at reason to a full blown war. Any damage taken taken by the host will not damage the Bones, though the GM should feel free to describe great gobs of flesh and gristle being rended, with the appropriate Perseverance Checks. Once the host has taken four wounds, the Bones will burst out, messily, and usually achieving another Perseverence Check. The Bones will usually then flee, since it is brittle after the inital "molting", the equivalent of one wound.)
Gro'n Bones initially resemble a distorted Human skeleton, though quickly grow to monstrous size and shape, often taking on a Draconian or Reptilian appearance. They are easily mistaken for Bone Golems (Orrorsh SB, pg. 100), but are much more intelligent in their actions. Once free of their host, Gro'n Bones will usually take up residence in graveyards, mine shafts, or the basements of spooky old houses, and are content to merely scare passerby with strange scratching noises, and the occasional lost pet. Unless of course they are crossed...