What souls are made of, what Alchemy spends, and what the world was seeded with. The single substance underneath every other system in the saga.
The rule
Everything in the world contains essence, in varying amounts (ch. 327).
A person's soul essence is the resource their abilities draw on. Alchemy is the deliberate bending of one's own essence into shapes it was never meant to take, which is why it is weaker than an innate ability of equivalent effect. See Alchemy and Soul abilities.
Where it came from
This is the Prologue's whole payload, and no later chapter states it outright.
Zero was a universe. He spent himself containing The End, and the fragment that survived was laid in his empty reality, where it dissolved outward and seeded everything that grew there (ch. 2).
"He's becoming everything he protected. Everything he loved." (Dream, ch. 2)
The chapter ends on "The 1 that became many."
So a world in which everything contains essence is a world made out of one person. Whether the story ever says this in as many words is an open question below, but the two facts sit either side of the same saga and fit exactly.
Crystals
Where essence pools in one place, it crystallises over long periods into mineral form. These residues occur naturally all over the world (ch. 327).
The cave chamber above Beckwith is scoured with enormous examples across its walls, and the arachnid living there grew to fifteen or twenty feet by feeding on them. See Beckwith.
That a living thing can grow by eating essence is worth noting on its own. It means essence is nourishment and not only fuel.
Density
Human souls are denser than monsterkind souls. Denser souls favour soulcraft; less dense souls bend more easily and so favour Alchemy. This is a tendency rather than a law.
It has a location, and it can be moved
Ch. 50 establishes a property nothing else in the saga tests: a person's essence is a thing that sits somewhere, and it can be taken somewhere else while the person stays put.
Magistrate Korren does it to Matta mid-fight.
KOR: I moved your soul essence, the magical core that grants your supernatural speed, to a pocket dimension roughly three thousand miles from here. You are now operating on purely physical capabilities.
Three things follow from it.
Essence is separable from the person. Matta is not weakened, injured, or drained. She is intact and her essence is elsewhere. What she loses is exactly the supernatural part of what she could do.
It is his standing method, not an improvisation: "This is how I defeat, rabbit. Strip away their essence, and they become remarkably... weak."
What is left underneath is real. She beats him on legs alone, which means the physical body is not merely a shell the essence animates.
Korren also claims his own power is not essence at all: "Your abilities were granted to you by forces beyond your control. Mine are inherent to my very nature." That is unverified, and it is the only claim in the saga that somebody has power from outside the redistribution.
Essence crystallises
Ch. 327, stated by Briar Colton inside a chamber under the hills above Beckwith.
BRR: That's crystallized essence. All of it.
BRR: do you understand how long that takes? Essence pools. It settles into places and it sits there and over a very long time it hardens into mineral. That's what those are. That is thousands of years of essence pooling in one room.
BRR: I've read about deposits like this. I've never heard of one this size. I don't think anyone has.
What it looks like:
great scoured seams of it running across the rock in every direction, formations the size of doors and the size of houses, some of them clustered in fans and some standing alone like pillars, and every single one of them caught Yseult's light and gave it back a little changed. Blue and green and a deep amber.
The chamber is at least two hundred feet from the shelf to the webbing, and its ceiling is not found until a light is pushed past a hundred and fifty feet.
What being near it feels like
Not unpleasant, which the chapter treats as the warning sign.
FNN: Does anyone else feel strange? TLN: Yes.
It was the opposite of unpleasant, which was somehow worse, a low steady warmth sitting behind his sternum, the sense of standing close to something enormous and entirely indifferent to him, and he found he did not want to move very much.
ELS: I want to look at it for the rest of my life and I also want to be outside. Both at once.
How large the chamber actually is, against the record
Ch. 329 gives the number ch. 327 was missing. Briar has read everything Solmere holds on crystallisation.
BRR: There is no recorded deposit anywhere in Maeris of that scale. Not one. [...] the largest confirmed find is a seam in the Iron Reach the length of this room. That chamber is a hundred and forty feet in and I could not see the far side of it.
The Iron Reach is Drennick, the province the metalwork and the mining come from, which makes the previous record a mining find in the country built on mining. This one is under a hill above a village of forty buildings.
Why this matters
Briar's failed device in ch. 275 to ch. 279 needed essence conduits, which cost him more coin than he had. This chamber is thousands of years of the raw material, undocumented, four hours from the province's capital.
Ch. 329 is where somebody decides what to do about that, and the decision is nothing. Briar works out what a find like this brings with it, and does not write it up.
BRR: A survey crew inside a month. A road inside a year, because you can't take that out on a footpath. [...] Odell doesn't sell the mill. Odell gets bought out by somebody who needs the water.
He keeps a thumb-sized piece for the workshop and asks Corin and Arthur to witness that he knows exactly what he is giving up. See Briar Colton.