So this guy conspired with demons, with foreign powers, betrayed the country, let villages get decimated by demonic forces and purposefully destroyed and hampered the Knights/Hero/Army whatever?
...And... It was so shocking that they covered everything up...
...But...
...They didn't kill him? They just... threw him in prison? N-N... N... No... You kill someone like that. Execute them. Whether it's done quietly or after a trial, it makes no difference. In this case, you apprehend him, find out what he's doing, off with his head in some secret dungeon and get rid of the body in such a way that no one will ever find out he's been executed. "Oh, what happened to Gordo? HE WENT MISSING. HAVEN'T SEEN HIM. Yeah, it's real fuckin' mysterious. Oh well. Too bad."
And, of course, he escapes on the SAME DAY that the hero he expelled from the Army shows up. Jesus Christ, author, FUCKING TRY FOR ONCE.