In the March of Inunciu ruled by a Demonarchy, a character must undergo a Trial by combat.
He has been framed by A collusion of guilds - uses resource control for leverage - and if found guilty of Spying, his punishment will be Baleful Polymorph. The prisoner permanently changes into a small animal. One of my favourite D&D modules is Castle Amber. Wouldn’t it be a neat twist for the central garden to be the prison of several banished beings changed into the form of small animals? If the subjects fail their second saving throw, they even gain animal intelligence, in effect becoming just another creature in the garden.

March - Similar to a county, but usually a borderland, ruled over by a marquis. Usually heavily fortified due to it's strategically important position.
Demonarchy - The rule or dominion of a demon.
Trial by combat - Fight to the death, winner is innocent