Necropolis – “Misfortune never comes alone” [Campaign 2 – Mission 1]

How do I complete the "Kilborn's List" mission?
And is it possible to complete it without capturing the second Castle? Difficulty level: Normal.

One of the computer-controlled heroes you need to defeat has this scroll; you need to give it to the skeletons, and then they will join you, and you will be able to build upgraded skeletons in the castles.