I was surprised by the ending myself and had made my DK too powerful. Fortunately I had a save game right before the end.

I went back to the save. Then I went to the battlefield and had my DK drop all of his inventory items, bought back all the skill points that had been invested in fighting skills, and put them into non-fighting skills. Then I armed the DK with a pot that I had laying around.

Men and nations do behave wisely, once all other alternatives have been exhasted