Will is the energy Souls radiate as the host experiences life and feels emotions, primarily will power, hence the name. It is potentially the most powerful force in the universe as it is able to warp reality and making the impossible not so. Will is an immaterial and invisible force that surrounds the Soul in a docile manner before fading away. It has been documented to have a tendency to protect its host and aid them in various, yet unpredictable ways. With enough practice, a host can manipulate their own Will and bend reality at their own command.
Will is a beings thoughts and emotions primarily their willpower, mixing with their souls natural life energy. The mental and spiritual energy intertwining creates the phenomena known as Will. As a byproduct of these two things its power fluctuates accordingly while it resides in the Hosts body. If the Hosts body has too much Will or if the Will resides idly for too long it dissipates into nothingness. Will passively regenerates based upon the users psyche but if the host strains himself he can regenerate it faster. This puts immense stress on themselves however.
Every being is different and thus has a different soul. Their Will can vary wildly from anyone but people in the same bloodline tend to have similar ones. These differences are called Sovereigns.
Will is able to control anything it comes to contact with up to and including the laws of nature if enough of it is present and used in the right manner. Basically it can literally accomplish any task if the Host has enough of it and the right Edict/Technique. It has been dubbed the Kings Power in many large Gatekeeper Circles as masters of Will are masters of the universe.