Consider Harmony, the most advanced spell in the Calm series. It works on targets up to level 25. You can boost that cutoff with two perks (the +calm one and the +person or +creature one, whichever's appropriate). There's also potions that make your Illusion spells +X% "more effective," although that could mean duration instead of level-boosting. Finally, you can give it a boost by dual-casting, although we don't know exactly how much that helps.

Picking an arbitrary number (since we, or at least I, don't know the exact strengths of the above boosts), let's say that you can make Harmony work on targets level 35 or under by doing all of the above. That still means that once you level past 35 and start seeing higher-level creatures, Harmony suddenly becomes worthless, and by extension, so do all the other Illusion spells except, say, Invisibility.

I have one core question, phrased three different ways:

1) Does anyone know the exact mechanics of Illusion spell level-boosting - what the perks do, what dual-casting does, etc?

2) Do enemy creatures continue to level up with you forever? I assume that the zombies you fight as a level 2 character are around level 2, does that scale all the way to level 30, 50, 80?

3) Are there any high-level, Illusion-casting players that could say what level they're at and which skills do/don't work (with which perks, do you dualcast them, etc)?

Thanks.

I don't believe it scales past 50, based on officially released info.

A lot of stuff probably doesn't even scale to 50, let alone past. I'm at 27 and using spells that are supposed to target level 14s. They still work.

5 of Illusions perks increase the levels that spells affect, and one 'increases the effectiveness" of all of the illusion spells, whatever that means.

Its vague enough that it might extend Illusion's effectiveness to 50, but it would take some pretty careful testing to be sure. Still, Todd stated that the game was designed to be a '1-50' game and that strongly implies that while Illusion might not scale up to 50, it certainly doesn't scale past it (and would not need to since enemies wouldn't).