So this may be a dumb question in that the answer may lie in the description, but I figured it was still worth asking.

The mastery Archaic Knowledge has a description that reads, "Your magical spells penetrate 15% of your target's magic resistance" Because sunfire capes do magic damage my question is, does this mastery have any affect on the magic penetration of the sunfire capes?

I can't promise you anything, but I think Sunfire ignores MR. Again, no guarantee.

If it ignored MR, it would be true damage. It's magic damage.

good question, kind of wondered about it myself!!! I propose a test of sorts, a practice game with sunfire cape on say amumu with and without archaic knowlege, start with sunfire cape with and without, test it, write down at least 10 numbers, then do an average, then add sorcerer boots during both trails and repeat the test

one test group
one control
should work

I've been curious about amumu's despair in the same fasion and how thats calculated. I assumed its true damage cause it says it drains enemies of % of max hp. But then some claim its magic damage, which would then imply its reduced by magic resist, but increased by spell penetration.

pretty sure its magic damage for realzies