Lifeforce

Lifeforce is the manifestation of a soul bound to the physical world. It is energy with near-limitless potential, able to be shaped into anything by a will strong enough. While that energy is normally locked tightly to the bond between the body and the soul, it can be forcibly separated and harnessed by trained mages, and rarely by supreme acts of will or desperation. The energy of Lifeforce is potent and even a small amount can accomplish amazing feats. But spending Lifeforce is also a draining experience since it is essentially the energy of one's soul, and the bond it maintains may be weakened. It is replenished on its own, but at a very slow rate. Child Races rarely access their Lifeforce directly for magic of any sort due to limited awareness of the methods of manipulating such energy. Elder Races are able to tap into Lifeforce more readily, but rarely choose to, utilizing the byproduct of Lifeforce instead; Mana.
Lifeforce has a more passive effect as well. When mana attempts to influence a field of Lifeforce, such as a mage trying to cast a Nature Magic spell targeting an unwilling person, the Lifeforce will resist that influence.

A soul bound to a living being is not a tangible force, existing simultaneously in- and outside of physical reality. As such, a soul cannot remain in the physical world without being bound to a body, nor can it interact directly with anything. Instead, the soul makes its presence felt through Lifeforce. The soul continuously generates energy simply by existing, as by its nature a soul is creation energy. That energy fills and adheres to the body the soul is bound to, maintaining and feeding the bond's requirements in the physical world. Without Lifeforce, the bond decays rapidly until the soul is unable to remain. At the same time, the bond can only hold so much Lifeforce but the soul continues to produce energy in the physical world. That excess energy either leaks away harmlessly and dissipates out of the physical world once more, or in the case of the Elder Races, remains loosely bound to the body as Mana.