The Dark Lord has been defeated by the hero and his party. Unfortunately for them, it was discovered that the Dark Lord had hidden a portion of his power, so that someone might find it, and use it to finish what he had started. You are a representative sent by the king to observe and aid the hero's party in their quest to find this hidden power. However, you might have the chance to be something greater.
In Rise of Corruption, you are the villain, and will have the chance to corrupt many characters, and eventually, the world. While there is combat, that is not the focus of the game. Instead, the focus is on slowly corrupting the characters and the world, and observing the changes that occur through each step until you have fully corrupted them.