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

Don't judge me too harshly)) I completed the mission by simply building up a large army of powerful creatures, but after reading the forum, I realized that there's an easier way: converting 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 convert them? I waited a week, thinking maybe they can only be converted on Mondays... But no... what should I do?
Help me out, friends, it's interesting:smile26: