The One Magic (一なる魔法 Ichinaru Mahō), also known as The Essence of 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, and it is 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 more about The One Magic, he gained a much more negative understanding of The One Magic, claiming that it was Darkness. 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.[
Layla Heartfilia and her daughter, Lucy Heartfilia, have an entirely different perspective, claiming that The One Magic is love.
According to Mard Geer Tartarus, when The One Magic was born, it eventually developed into many different kinds of Magic, including Curses.
Connection with Zeref
In his quest for The One Magic, Hades related The One Magic with Zeref, thinking he was the key to obtaining such power. The degree of truth to this is so far unknown.