

No, you don't really need your nova or AEO spells but they add more flexibility to the necromancy sigil. So, your fire-damage undead minion will do more damage based on you necromancy destruction numbers and not your fire numbers. The strength of your minion is directly tied to your necromancy stats. Yes, you need to upgrade you necro spells.

What does this retaining thing mean when you have more than three? Is this kind of a storage which of the three you can keep? Why doesn't it last longer? Weird stuff. Even more so when the enemies are pesky undead too. It's really difficult to tell them apart. Why is nobody telling me what they can do and stuff and why do they look so similar?

What is the difference between the undead you can summon? Why do you need a Nova or an AOE in necromancy? (except maybe bone armour) Khadgar42: Do you need to upgrade your necromantic spells?
