You can capture your opponent's Hero by achieving a sizable victory over them in battle. This also means your own Hero will be captured by your opponent if you suffer a sizable loss.
[Hero Capture Conditions]
A Hero won't be captured if the target Castle or March doesn't have one.
Heroes can be captured after their Citadel reaches level 13.
You can't capture your opponent's Hero if your Castle's Prison is not open.
Only one Hero, who is appointed as an Aristocrat, can be captured at a time.
Heroes you capture are detained in your Prison, and you can release them at any time.
If a Lord who is imprisoning a Hero loses a battle at their Castle or uses a Peace Shield, all the Heroes they have imprisoned are released.
All imprisoned Heroes will attempt to escape once the Prison reaches maximum capacity.
The probability to escape increases with each excess prisoner.
Lords who have their Heroes captured can post a bounty on the Lord who captured their Hero. High bounties compel other Lords to collect on them by freeing the captured hero.