Spirits

Every being, human or not, has a spirit. It’s the energy inside them that allows them to live, and in the human case, do magic. Magic happens when someone with a large enough spirit (“size”, or rather the power of the spirit varies from person to person, and can sometimes be increased by adding the spirit of something else with the help of a magic crystal) transforms part of their spirit into magic energy. Your spirit usually cant be depleted using magic, but it can weaken for a short period of time. When someone dies, only their spirit is left, which will grow conscious and then it will in most cases travel to Osturos, where they live another life before they are reborn. People worship the power of the spirit through Spiritpractice. 
(A spirit and a Spiritbeing are two different things)