1. We attack 50-99 skeletons guarding the prison.
2. 61 skeletons, including an upgraded stack of 12 skeletons.
3. We kill all stacks except for the upgraded one.
4. The upgraded stack kills our Knight, and the hero leaves us.
5. The prison guards also disappear along with our hero.
This is how SOD works.
P.S. If you run away from the upgraded stack with your hero, the story will repeat – the guards will disappear.