Place a Necromancer hero on each pentagram near the Knight Castles, except for Arantir. I think you can't hire more than three, and the fourth should be Ornella. Just take away all her artifacts and troops beforehand, as all these heroes will die after the ritual. Then, cross the bridge and take the last castle. The army there isn't very large, but the enemy hero has very brutal characteristics and combat morale with luck. So, take a larger army to avoid risking it.

P.S. Moderators, I think this topic should be divided by scenarios. In a month or two, there will be so many posts here that it will be impossible to find anything.