This is the energy left behind by living things when they die. Small amounts also exists within their bodies while they are alive.
There are two types of essence.
Lebenflus is the essence left by Fifth Realm life forms. Everything from humans to microorganisms.
Human Mancers use this essence as “fuel” for their spells.
The other type of essence is the Voldeflus of The Hub. This essence has been flowing from a dying void entity that has existed since before the universe.
The god-like entities that exist within The Hub were created by, and are masters of, Voldeflus.