Necromancers – “Assault” [Campaign 3, Mission 2]

Thanks for the tip, I just checked the Necromancer skill tree and saw that Summon isn't the top skill, and Marcellus was initially given the leadership skill... And yes, I'm leveling up Darkness, but I don't have enough mana, even with the Necromancer bonus. I cast one spell, and the enemies attack another target...