The Incubus King

Though the ostensible main villain, the Incubus King seems likely to play a smaller role in the plot. Though he holds one of the biggest Soul Shards, he's not necessarily the most powerful person with one. He's definitely not one of the most competent, since he's dangerously stupid and ruled by impulse.

Story
His arrival on Arclent (from Renthnor) completely changed the continent, setting off the war and destroying multiple nations. Since then, he's grown complacent and mostly sits in his lava castle. It seems like he leaves most of the management of his empire to Iris and inspires very little loyalty in anyone not in his direct presence.

So far he's displayed several fetishes that are on the odd side, even for succubi. These include the "crownjob" and something that involved a giant chess set made of feces.

He insists that he is the only one and demands everyone call him Incubus King. During the first meeting of the Gathering he proclaims himself to be the:

The real powers have their own titles and don't mind that much indulging him, to a point of course, but some of the lesser ones are pissed about it.

During the Gathering, Yarra experiences the full scope of his power and only the timely intervention of both Esthera and Estaven allows her to leave the reunion as a free succubus and not as some kind of sexy zombie.

But in one sense he is exceptional, because he has two amazing subordinates that helped him: Iris - not dedicated to him personally, but to the people of his domains - and Riala - a mage of great power and knowledge, used as his personal errand girl for humiliating tasks. Of course, he doesn't appreciate them any more that he does his kitchen's servants. Riala has betrayed him already by giving Simon his soul shard, while Iris is likely to betray him as soon as she believes it's in her people's best interest to do so. Riala has already been caught and punished in some way that seems to have broken her will, so she may actually be more of an asset than ever.

Wiki trivia
As coined by Argenten we've begun referring to him on the wiki and forums as: "Arsehole of Arclent" to help distinguish him from all the other Incubus Kings. (Because it fits him pretty well and cuts the confusion about what we're talking about down quite a bit.)