Foreskin training for captive hero

The Demon Lord's army is at war with the intention of dominating the entire world.
A hero fights against them to protect the world,but he is captured by the enemy just one step away from defeating her.

The Demon Lord's army is on the verge of destruction. They target the whereabouts of the enemy princess as a way to rise from the ashes.
However, the hero never speaks of it. She is also his fiance.
He tells the Demon Lord that he will endure, trusting in the help of his friends.

Her ambition shattered, the Demon Lord decides to inflict torment on the hero in order to take her anger out on him.

What is the method the Demon Lord chooses to inflict the greatest humiliation on the hero......?