Energy. According to the Law of conservation of energy, energy cannot be created nor can it be destroyed- simply converted from one form to another. Chemical energy. Potential energy. Electrical energy. Nuclear Energy. Kinetic. Thermal. The list goes on- a quick Google search will reveal them. 
Except one. Magical energy. Or rather, Dark Energy.
I am developing a system of magic for my land(s) that is based off of the idea that magic is nothing more than converting dark energy into another form. Want to knock that group of archers off the ledge? Kinetic energy should do the trick. Want to start a fire? A number of options there.
But what is dark energy and where does it come from? 
Dark energy, like its counterpart Light energy, is supernatural. The energy of creation...of the spoken word of God...of the Holy Spirit...is Light. The energies of demons and the fires of hell are Dark. The two are nearly one in the same. Only God should wield Light energy. It is His right and privileged as the Creator. One can be a vessel for Light energy through faith, but to take light energy for one's self would corrupt it into dark energy. This is why the demons posses dark energy. Before the fall of Lucifer, they were vessels of light energy as angels. After the fall, they took that energy as their own and corrupted it.
Being outside of the realm of Natural Law, Dark Energy is limitless in that it can be collected, stored and brought from one world (namely the demon world) to another. The demonic stores of energy are beyond comprehension, and thus virtually limitless.
This energy must be stored- it is similar in nature to potential energy. It must be stored in gems. Few are capable of withstanding the forces of the energy. 
Demantoid is the best known 'conductor' of this energy and is most widely used. When it is full of dark energy, its green color pulses, like a breathing ember.
"Spells" are simply commands used to apply the energy and change its form. There is no known spell to change other forms of energy into dark/light energy- such an act is divine. Spells are limited by the amount of energy. If there is not enough energy to complete the spell, the spell will fail, often half finished and with unknown consequences.
As previously stated, as soon as light energy is taken for one's own use, it is dark. Therefore, magic cannot be used morally- it is always an evil and a rebellion against the will of God. It is addictive. It corrupts the soul and mind and slowly kills the conscience. Even if used for 'good', it is still a moral wrong.
That is all for now. Any questions, foreseen problems, or comments?