Estus

Estus is a form of life energy that all individuals produce to some degree. Through various methods, estus can be controlled and manipulated to create a supernatural effect known as magic. Estus is ordinarily not visible unless it is manifested in large amounts or highly concentrated. It is rarely seen due to mortals having a natural limit on the amount of estus they can control at a single given time.

Origin
Estus originally belonged to the Illyriads. When the Seed of Vitality split, it expelled Estus into the world which manifested into the Illyriads. Malchor created mankind with Estus, using it to manifest his creations by giving them the life force of the gods. Now, all creatures that descend from mankind of the Illyriads have estus as a vital life force.