This is a datapack integrated into a mod for convenience. It adds Iron's Spells & Spellbooks magical mobs in the pool of potential invaders of Apotheosis.
In config/apotheosis/apotheosis.cfg you have to set B:"Boss Glowing On Spawn"=false.
If you don't do that, magical mobs will spawn without their texture (still trying to find a way to fix this without disabling glowing for all invaders).