The Fable-Wyrm, Eater of Legends
All stories have power. This seems like an empty platitude, but the finest scholars of the world, in particular divinity and bardic magic, have discovered that myths, especially archetypal, repeated ones, carry a measure of memetic power enforced by the minds and spirits who find importance in them. Some have also theorized that these stories are also something of a life form in their own right, and gods are symbiotes with them. (Religion DC 20)
Said scholars have also found evidence that suggest some gods are not symbiotic at all, but parasites. By inserting themselves into the legends of other gods, they force those gods' worshipers to acknowledge their own existence, and thus leech off the mimetic power without actually creating and supporting myths of their own. While it is believed the vast majority of these gods are actually symbiotes with at least a couple myths (for example, Grummush and the stories orcs have of their creation, with his parasitism being the presence of orcs in other stories), there are rumors of at least one entity that absorbs mythic resonance entirely through parasitism. This being, sometimes known as the Fable-Wyrm, is said to favor stories of Elder Evils, as the gods would resent one of their own contributing absolutely nothing at all. (DC 25)
In fact, the Fable-Wyrm is quite real, but he is not a god. Rather, his true nature is that of an entity whose name is best represented in mortal languages as Mnlqbf, a "dragon" from a strange universe in which the basic laws of reality are not mathematics, but literature. The Eater of Legends is a member of a species of antagonists, specifically those taking after mighty guardian creatures to feed off the stories of hoards and those who would claim them. Due to a genetic mutation, however, the Fable-Wyrm could not properly digest the legends of wealth, but derived extra nutrition from stories of heroes. He is said to have come to this plane to seek a better food source. (DC 30)
While not an intentionally malevolent entity, the Fable-Wyrm is callous, uncaring, and oblivious. It is by his doing that several Elder Evils have been unleashed, as the stories created by those who would reseal or banish them is enough to sate him for a decade (and to be frank, he likes watching explosions). If the world must die to feed his appetite, so be it; there are millions of other worlds out there, and besides, noble tragedy has a unique and potent taste. (DC 33).
The Fable-Wyrm's desire and goal is to provoke meaningful conflict, preferably that which results in acts of incredible skill and courage, since the memetic energy produced by those stories is his preferred diet. Other than that, the Eater of Legends seeks entertainment, and will regularly manipulate things to cause mishaps, troubles, and general hilarity for his amusement. He's always open to mortal ideas of what might cause those things, whether the mortals wittingly inform him, or not.
The Fable-Wyrm was never imprisoned, and is in fact effectively omnipresent. He's wherever the story needs him to be at the moment, and when one is involved in his legends, that means his invisible, intangible blue form is everywhere. He does not have a true Sign, although many may believe otherwise as he manipulates things to be more entertaining and nourishing (ie, hilariously wrong).
Besides the normal qualities of an Elder Evil, the Eater of Legends has a unique ability:
In A Story, the Teller is God
Within the bounds of one of his plots, the Fable-Wyrm treats the entire area as divinely morphic, with himself as the divinity in question. He may even do things that change causality or seemingly random events, even create new people on the spot. The only way to avoid this is for a subject of his tales to declare that it isn't fun any more and walk off, excusing themselves from the story permanently.
If you are trying to punch the Fable-Wyrm, either you have problems or he has gone way, way too far. Either way, he's not really supposed to be your antagonist, despite what he thinks.