Gehenna has been a bit part player in one of my games, albeit a high level game. I have not listed him/it in the characters section as I dont feel he/it is up to my usual calibre of submission. However, since you happened to mention a bone demon, and I happen to have one laying around, I thought you could use him/it. On to the beast...
Gehenna was created by accident some millienia ago by a less than cautious mage. The mage, then fascinated with the nascent arts of animation and demonic possession created a demon powered bone golem. As expected, the bone golem proved to be a terrible opponent, slaughtering foes and absorbing their osseous mass into his own. The golem grew, and when it judged its power great enough, it attacked and destroyed its creator, absorbing his sorcerous might as well as bones into its body.
Unbound, the demon rapidly increased its size and power, assimilating entire villages, herds of livestock and even stalking the isolated young dragon. At the zenith of its power, Gehenna was easily mistaken for a dracolich as it chose to use the head of a slain dragon as its own, and moved about with adraconic form for the fear it caused.
It was a rare coalition of elven magi, dragons, and orcish necromancers who finally banished Gehenna, but at great cost. Many of the coalition members were slain and absorbed by the demon before it was banished to the Hell of Dry bones. Trapped within a prison hell, the demon lord raged, and his rage created mountains of bones, rainstorms of teeth and knucklebones, and sandstorms of pulverized bones.
Now trapped, only a gifted summoner can create a conduit for hich Gehenna can touch the real world. A supremely difficult ritual requiring large sacrifices can create a conduit large enough to release the greater demon back into creation. Gehenna is always seeking a willing servant that may one day release him from his realm of bondage.