Necromancer – “Invasion” [Campaign 3, Mission 3]

That makes perfect sense, but since I'm hesitant to mix troops due to morale drops, I didn't have the option to try out the permanent map ("live hero") feature with an undead unit – I simply don't include them in the army of a living hero. There are usually only living units there. I'll need to keep that in mind; it might be useful.