So, I made a new character and am using illusion with him, and it's been a lot of fun so far. But I've noticed that the level increases to the spells seem to be a bit static, the perks increase it by a fixed number and the skill caps at 100. So there would be logically a max level of creatures where the illusion spells work. So I have a few questions, as I can't seem to find this information anywhere:

1-Is there a certain level where the illusion spells will "cap" and simply stop working because of monster level scaling? Or is there a hidden scaling for the spells somewhere that I have missed?

2-If so, what is the max level? Because my build will cap at around level 50, and it would be a shame if my illusion spells didn't work against level 50 monsters...

Thanks in advance.

Nope.

Monsters stop scaling max monster level is like 52ish your spells will go past that by a large margin as long as you dual cast and have the rest of the perks.

Oh, nice. Thanks.

Is it the same for the master spells, though? As you can't dual cast them?

If you max out your illusion you will be able to control most anything that stands in your way.

No, Master Spells cannot be dual cast and are basically useless. For example, there as some monster that are too high for a maxed out perk tree Harmony to affect - yet a dual cast Pacify will be able to handle them.

Dual cast effectively doubles the level of each spell.