No one knows why Jon decided to be king, but he up and ate the local royal family. He had lived many years as a recluse in his castle up north, a shape-changing ogre whom no one dared disturb. Some say a young noble stole a goose from him, or some magic beans. Whatever the transgression, Jon Bonemeal visited a brutal and bloody revenge.
Taking the family’s keep as his new home, Jon Bonemeal crowned himself King Jon of Argl. He had letters written to all the nobles of Argl, demanding their allegiance. Those who refused he attacked personally, toppling castle walls, throwing men through gates and eating the aristocracy. He managed to get the northern third of the island to pledge fealty, but discovered that ranging too far from his throne lead to an exasperating number of petty rebellions to put down.
Jon still claims to be King of All Argl Lands and takes the form of peasants, children and beasts to spy on his subjects. Small infractions are punished brutally and vividly, so stories circulate and keep the others in line.
King Jon is much hated by everyone, naturally. He is also much feared, and this fear has held most on check. He is surrounded by sycophants and cowards of human stock. Of his attempts to produce an heir, the less said, the better.
In your campaign, put him several levels over the party and have them meet a cousin of the noble family Jon killed. Perhaps they would be rewarded for helping him…