"The Will of Ashan" - "Heart of Darkness" [2nd campaign, 5th mission]

Don't be too harsh on me)) I completed the mission by simply building up a powerful army with advanced creatures, but after reading the forum, I realized that a easier way is to transform other creatures (e.g., from captured castles) into undead.
The corresponding building is in the castle, people from other castles arrive, but how do I transform them? I waited a week, thinking maybe they can only be transformed on Mondays... But no... what should I do?
Help me out, friends, it's interesting:smile26: