I booted my computer and now this is what I'm looking at.
It's like my system font was set to Marlett.
Furthermore, certain apps are unaffected, such as Chrome, Settings, (most of) TeamViewer, etc.
What did I do, and more importantly, how do I fix it?
Edit: With some help from @MC10, I was able to determine that installing a TTC font file is what borked my install. That being said, I also can't perform a system restore because during the upgrade process from 8.1, System Protection was disabled on all of my drives.