In general, the magic system is based around FFXIV's magic system on its base level - the idea that Ether is a major part of a person's composition and because it exists within everyone and every thing, anyone has the capability to learn to manipulate it, with the ways they do it based around their proficiency, capacity, and general capabilities.

Because of this, there are any number of schools that train the use of Ether, so the aesthetics of spellcasting can match whatever style you're going for, there would absolutely be incantations that reference the Old Gods within some spellcasting traditions.