The One Magic

The One Magic or The Magic of One is the original source of all Magic. Every form of Magic as it is today in the series descends from this source.

Description
While The One Magic is the "all-father" of Magic, in present day it is little more than a concept. Its roots have never been fully disclosed and given the many forms that Magic has in the series, The One Magic is therefore very complex and difficult to understand for even the greatest Wizards, and is hence subject to many different interpretations and understandings that have yet to see a rightful explanation. Even with such ambiguety, The One Magic does seem to share a common opinion amongst Wizards, that is that being the source of all Magical Power, it is also an extremelly potent power.

Interpretations
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 in a quest to find more about The One Magic, he had 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, normal humans would live in hell IF they were capable of living at all.

Layla and Lucy have another much more different perspective, claiming that The One Magic is love.

Connection with Zeref
In his quest for The One Magic, Hades related The One Magic with Zeref thinking he was the key to obtain such power. The degree of truth of this is so far unknown as while Grimoire Heart was successful in "awakening" Zeref, the methods they thought were essential for such goal, were in fact proven wrong by Zeref himself. Despite the fact that Zeref is indeed an extremelly powerful Wizard capable of killing anything instantly and creating life out of nothing, demonstrating great power in his "awakening", however it was not as close to what is defined as The One Magic. Even in the X791 arc 7 years after the destruction of Tenrou Island there has to be seen any substantial change in the world.