Well, if you wanted to be technical without any cultural connotation, something like that could work - although is also maybe not distinct enough, or accurate to what mystics actually strive for. But for an RPG? Sure. But I think a more accurate term is probably vital energy, because it specifies an aspect of "mystic reality" rather than glomming them all into one.
To illustrate, look at the Yogic Koshas - which are five sheaths or layers of the human being.
The first is the annamaya-kosha, which literally means "food sheath." It is our physical body - and the only kosha that is recognized by modern science (which is why I'm suggesting that equating chi or prana with physical breath is reductive).
The second is the pranamaya-kosha, which translates as "breath sheath." Prana is vital, subtle energy with flows through our body - it is essentially our life force.
The third is manomaya-kosha, or "mental sheath" - our thoughts and emotions.
The fourth is vijnanamaya-kosha or "wisdom sheath" - and harder to understand from a modern, Western perspective, but somewhat equates with deeper intuition and wisdom. To differentiate it, we could say that the manomaya-kosha is the thoughts that we have and are aware of, like thinking about your day or life plans, while vijananamaya-kosha is more like a deeper sense of knowingness or insight.
The fifth is anandamaya-kosha or "bliss sheath," which is the deep joy and bliss that is the true nature of our beingness.
Now all five of these are sheaths or layers around the true Self, or Atman--which in turn is the individualized version of Brahman; the common analogy is like a drop of the ocean. The "substance" is the same, but one is looked as an an individual self, the other as universal. One could imagine this as an aperture within all of us to the infinite, eternal, and ineffable "divine being." But the key is that we experience the five sheaths as aspects of our being, but who we are is that which is experiencing and aware of them.
So in one way, none of the sheaths are "essence," but the layers around our true essence, and a mystic is someone who seeks recognition of their true nature as Atman.
So again, I would suggest "vital energy" for prana (or chi), or perhaps "life force" or "vital force." Henri Bergson's term elan vital is somewhat similar, and probably his understanding of the same energetic element of the human being: the subtler layer of life force that permeates the physical, but is not the physical and cannot be reduced to the physical.
One mistake modern people (not just Westerners) make is equating these sheaths with subtler aspects of the physical world, which is essentially reducing the five koshas to one: the annamaya-kosha. Now whether or not that is objectively true is another matter, but it is a misunderstanding of these systems as they are intended to be understood. Four of the five sheaths are not material, not of matter - even subtle matter (or energy). Meaning, it is not technically accurate to consider prana to a subtler physical energy, like the various forces of the scientific paradigm. It is actually a different layer or "dimension" of existence. And each layer is even more subtle; they interact, but are distinct from each other.
en.wikipedia.org
