Help me understand the mechanics of the Death Knight; I've tried all the options, but nothing happens.

Pay attention to the skeleton in the castle courtyard and carefully read the note you found with him. And you will be happy:D

Added after 2 hours 24 minutes
Remembering Remix's favorite phrase - for a true player, there is only one difficulty - the Hero - I replayed the map - 88 days (caught the log and bought the boots of swiftness). It seemed easier than before with the Warrior - I mean the battles, not the RPG elements of the map. I was surprised, but then I realized - with the right development, the overall strength of the enemy army is not so important, and at the beginning, you can't put out too many units - otherwise, you won't be able to pass it, no matter how skilled you are.
Thank you again to the author for the wonderful map! By the way, there were no crashes again...