RPGstats or PlayerEx gives you buffs like "Mage DMG increase". It doesn't seem to be compatible. Or is it? It should be, so more mage related things connect together. I feel like Magic Missle is also not affected by any dmg increase or decrease, but Solar Strike is. Why is that? And can you look into that?

that's because only Solar Strike is a unique damage type. i most likely wont make any changes to current Arcanus, as im working on a sequel, Arcanus: Continuum
https://github.com/CammiePone/Arcanus/blob/1.19-dev/src/main/java/dev/cammiescorner/arcanus/core/registry/ModDamageSource.java

Wait what?? A sequel? You're godsent and I'm hyped. That's wonderful news (when does it come out :o), but I somehow really want to know how certain effects interact with certain damage types. E.g. magic missle and solar strike. What defines them?