Unlike Prism, it does not achieve harmony with light and dark, but rather overcharges the element being used. If we proceed in this way, we can use three types of Light Energy to fill the "Transcendence energy." For example, if we want the Void Subclass to use "Transcendence", we must use Arc, Solar, and Void types of damage to fill the energy, and none of the three are missing. If one type of damage is missing, "Transcendence" cannot be used.
Or it can be achieve harmony with the opposite element. For example, using Strand type damage to fill the "Transcendence Energy" of the Void Subclass.
Anyway, it's just an idea.
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The other subclasses could certainly use something similar to transcendence. How the bar would get filled is kind of hard to figure out though because the other classes might be more interesting if they focused all on compounding the power of their respective element rather than combining them. Prismatic has got the combining thing covered but filling the bar could also work if you stuck to either light or dark elements to fill it. Then there's also the problem of how weapons are distributed so you'd almost have to use an element that's the opposite of your subclass. Almost makes me think something needs to happen with how our inventory is distributed but I wouldn't know what.