The Six Heroes slew the rightful ruler of all creation Demon Lord Karnas, saving the continent from the forces of Chaos in the name of Solansia. That should have been the end of things, however...
Karnas had children before he died; a son as mighty as he was cruel, and a daughter as scheming as she was brilliant. The two of them inherited no mighty armies, but plotted anew in the ruins of his old keep. If this world had heroes mighty enough to slay such a force of nature, then surely they would be unstoppable with such power on their side, and the world would be theirs...
They did not aim for the mightiest hero, nor the most brilliant, nor the best mage; instead, they turned their eyes to the most resourceful, the most scheming, the one whom the nobles had used up and cast aside as a "dirt general" whose rise had been a violation of Solansia's order. They kidnapped his wife, and bad him to serve them in return for both their lives.
What will become of them, you ask? Will the hero become a monster, or will he cling to "doing the right thing"; will his wife balm his conscience, or be twisted by the castle's evils? Well, that's all up to you, isn't it...
(Apologies if that was a bit melodramatic, I wanted it to serve as a good hook. Though, I could have made it smoother with twenty sentences... Also, apologies for the "rightful ruler of all creation" joke; it'll make sense after Skordred's events.)