Aorrus
"Speak, Shout or Scream what you like. Only a thought can ever be true."
Chaotic/(Good, Neutral, or Evil)
God of dreams, inspiration, and intoxication(as well as herbalism and medicine) outlawed on Ortho, with a semi-secret prescence on that prime and several strong cults found in the colonies of Ortho.
Aorrus manifests in many forms, taking on mixtures and features of different genders and different species. He, she or it is said never to appear the same way twice. The god's history with the Harmonium and Ortho is long and convoluted. Long before the Harmonium, Aorrus served as a mediator deity between the mortals and mortal world and the gods, as well as helping the gods communicate and to mediate disputes between each other. Indeed, Aorrus is in part responsible for bringing some of the inspiration to the first party that founded the Harmonium. He is also credited for a multitude of herbal remedies and folk cures that are widely used throughout Ortho. It is said that he was one of the first gods that was called for to be banned and one of the last to officially be banned outright. Compounding the problem are several partly successful attempts to convert worshipers and cults of Aorrus to a more lawful alignment. These attempts have left several ideas circulating throughout Ortho including the diety with the Old and New gods of the Harmonium. The god's present is subtlely woven in with the grain of the folkways of Ortho and is widely regarded as a troublesome stain, difficult to remove.
Writing Note: The influence of Aorrus is similar to the influence of Druidism on the Cristian faiths across Europe. Similar to why we have Cristmass trees, and other traditions which were originally Druidic and appropriated into the culture is how much of Aorrus's influence is felt. His few active worshipers and clerics would be treated much as witches would be in Cristian Europe. Publicly denounced but often sought when conventional means fail.
2nd Note: I am introducing my ideas for this god here because I am working on a couple of sites related to him.
Sounds like a variation on Dionysus - fairly neat, though I don't know how important he would be especially after a concentrated effort to remove the Lords of Chaos and any other minor deities like this one. The Harmonium influence is extremely potent (the populace truly believes, it's not that they patiently tolerate whatever religion of the week their rulers believe in), so the cult for him is guaranteed to be more underground than you've implied here - though that may be what you're working with and I'm just not reading it right. If you can spare some of those creative juices on some of the bigger holes in the setting I'd love to see those works too.