Going for an Illusion/Archer build, BUT..

[ShamWow]

I know magic doesn't exactly scale well and by end game destruction sucks. I'm looking up Illusion spells and the most powerful one being mayhem says that npcs up to level 25 will attack each other. Level 25 isn't exactly high, and will pass that in no time. Does that mean that by the time I'm around level 30 or 35 Illusion spells won't work? That can't be right. Can someone please clear this up for me?

[DrNewcenstein]

That's basically what it means. Spells stop working once targets exceed the maximum limit of the spell. It doesn't help that most spells have weak percentages to begin with. There are some things you can do like the perk tree, but really they crippled magic in Skyrim.

[Rez Elwin]

There are perks that can help but you still won't be affecting targets past like level 40. Though you do need to account for enemeny scaling as well, bandits for example stop leveling at 28, so the spell will always affect them. Where as Dragon Priests at level of 50 it will never affect.

The higher you are the less useful it becomes bit there are some scenarios it can be used.
You have a room full or draugr? Well only Deathlords and Death Overlords will resist it, as long as there are some weaker ones around it will still work on most of them.

[HazakTheMad]

With all perks, to keep up you need to start to invest in fortify potions, it will continue to be relevant if you stock up on fortify illusion potions.

[Girl-y-Girl]

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[Nixonator]

Overcharge and enchanting or alchemy fortify skill makes all magic usable even in late game on legendary difficulty.