Shadow of Camelot
Mercia is a large, relatively peaceful kingdom situated in the middle of the island.
Penda the Benevolent is current ruler, in Sheffield, the seat of Mercian kings. While sated with seemingly immense power, he is perfectly content to let the peasants in the countryside fall victim to raiding bands. He claims to have vague ties to Uther Pendragon, and espouses the virtues of King Arthur.
While not interested in religion, pagan or otherwise, he allows his people freedom to worship as they choose. He has built a few monasteries, abbeys, and cathedrals, but most of his focus is on feasting, song, and jousts. If a bard or a cleric amuses him highly enough, they can be certain of walking away richer.
But the forests of this region have seen the build up of many malicious cults, celebrating the darker powers that now scurry unchecked throughout the land. They rule over various towns quietly, biding their time until the great Penda dies, and his powers crumble.