I'll be retiring my warrior soon, and will be starting a nightblade soon who makes heavy use of Illusion. So, I need to know how useful Illusion will be in the later stages of the game. For example, take frenzy, which affects enemies up to lvl 14. Therefore a fully perked frenzy will affect level 26 enemies (14+12). Assuming dual casting doubles this level, am I correct in assuming that it will affect level 52 enemies? Or is it actually (14X2)+12, giving 40?

Basically, what I'm asking is will the perk modifiers take effect after the dual casting, or will they be factored in the dual cast effect? You PC users will probably be able to check this with ease, so I hope one of you takes the time to do this for me. Thanks in advance.

Yes, dual cast effects the level. It will be heightened with dual cast. Dont forget invisibility while you are at it.

I already knew that. It's the way the affect is calculated that I'm unsure of.

Can anyone help me here?

From what I read in another thread it would affect 50+

So the doubling from double cast factors in after your perks? That's great!
One more question, however: I just found that vampirism makes your illusion spells 25% more powerful; how is this calculated?

i dont know how its calculated, but it works on higher level enemies, especially whith the Animage and Kindred Mage perks... i use it both with my mage and my archer

On UESP, it says vampirism can be used to reach lvl 70+ illusion spells, so it must be pretty good. Don't know if I want to be a vampire though...

I have not seen the numbers but when I dual cast nothing resists my spells unless they resist them via magic resist or something. I never get a this enemy is too powerful statement any more. Thing is you really have to keep the skill leveled and perked to keep up with level scaling, the dual cast perk is a great place to start given how it doubles the effective levels. It is a very swingy skill, perked and skilled up virtually all powerful, not perked virtually useless outside boosting your stealth for your nightblade.

With vampirism you can have your illusion spells effect NPCs and monsters level 70+ so no worries as to endgame

I know it doesn't work on dragons, but what about dragon priests? Imagine calming a dragon priest and getting a backstab animation on it!
Also, do you guys think that I should become a vampire, even though I'm only going to play the nightblade to around level 35-40 or so?

Without vampirism I know your Illusions affect Dragon Priests with max perks and dual cast; so 50+. Dont forget that there is Hystaria, Mayhem etc after Frenzy, Pacify, etc. So you have a whole new grade that pushes levels up.

I need help in the Illusion skills as well... as far as I know once you hit lvl 90 with Illusion you get an "http://elderscrolls.wikia.com/wiki/Illusion_Ritual_Spell" quest from http://elderscrolls.wikia.com/wiki/Dervis_Neloren. I'm already 92 with Illusion and I'm not getting the quest at all from him. Can someone tell me what's going on?

I think it can bug out - didn't get it till 100

Also, frenzy and pacify are aoe, right?

Yes, they are.

Not a large one, but they will effect multiple opponents. Not like the next level (like mayhem, harmony and hysteria) that sort of covers everything you can see, but still a quite large area.

I currently use a straight up illusionist Character, every Perk in the tree, every spell I can get my hands on.

Illusion in my mind, is the best magic school.
Its ridiculous at high level game content.

Many times while playing my first character through I came upon situations that I couldnt handle, too many things, too strong.
With illusion its simple, make them fight each other. once one is dead you can calm the other, figure out what to do and proceed.

Trust me when I say, illusion is well worth it.
(if you want a quick level ,buy muffle and cast it a few times)

Excactly, Avidus.

Actually to an extent where the game on master diff. has now, for me, become too easy to be fun.

Cast frenzy, go invis = winning.

Or actually just go invis and calm as an "oh-crap".

Can literally sprint thru the entire game. (just not killing anything tho)

EDIT: actually I had the same thing with my first char, an archer - so OP... too OP