The One Magic (一なる魔法 Ichinaru Mahō), also known as The Essence of Magic, Primordial Magic or The Magic of One, is what is thought by some to be the original source of all Magic. Those people believe that every form of Magic as it is today descends from this source.
While The One Magic is supposedly the "all-father" of Magic, in present day it is little more than a concept. Its roots have never been fully disclosed and there are many different interpretations and theories to its actual nature. Even with such ambiguity, those who believe in The One Magic do seem to share a common belief about it: that it is the source of all Magic Power, an omnipotent power.
So far, few interpretations have been given in the series about what truly is The One Magic. Both Makarov and Hades at one point, believed that Magic simply comes in many ways, and that it does not have one concept but that Magic is universal.
However, as Hades left on a quest to find out more about The One Magic, he gained a much more negative understanding of it, at one point claiming that it was darkness itself. He thus began to relate The One Magic with Zeref, thinking that the Black Wizard would be the key to obtain it. So much did his point of view change that he, and, by extension, Grimoire Heart, thought that in a world where The One Magic were to exist, humans incapable of using Magic would be living in a place comparable to hell, if they were even capable of living at all.