There is little that can be properly described that can rival the knowledge of a 'True Name'. Families of humans have dedicated generations of their lives to learning a single one, and if any have succeeded they have not spread the knowledge of such. Why would they? Ultimate power is offered to those that know the 'True Name' of an aspect of reality, a person, an item. Rivaled only by another, that holds the same knowledge.
— "The Seer", Merlin
True Names
A True Name is the absolute name of an aspect of the world. Be it fire or light, a forest, even down to a stalk of grass or even a person. The power of this knowledge allows absolute dominance and control over whatever has been named, and is considered some of the most sacred, priceless, and overwhelming knowledge to exist within the world. Though many mortals have coined them as such, no true name is ever in actuality a word, name or even something that can be spoken or written in the same way that language can.
Learning the Truth
Learning a True Name is a painstaking process that requires an individual to dedicate everything to the aspect of reality, whatever it may be. It can take hundreds of years of effort to reach even a single name and requires the most intimate study of whatever it may be in all methods, and more than that, a perfect understanding of what makes it what it is.
Due to this, knowledge of True Names are considered the most forbidden of fruit, and undisputably priceless knowledge. Should one dedicate enough time and understanding to something, its True Name will eventually come to them. Sometimes in a dream, or at other points simply manifest within their mind in moments of peace and contemplation.
The only way to acquire a True Name without doing this intimate study and understanding, is by simply learning it from someone else. As understanding is not required to know a True Name at all, however, to use a True Name to its maximum potential requires that knowledge. As with all magic and skills, shortcuts will grant tremendous power, but not of the same level that true dedication will, and often with pitfalls as abuse of a True Name without understanding will lead to reality itself eventually stomping you out for your abuse.
Still, despite this, it does not stop many from attempting to learn True Names through underhanded methods, and as such, it is rare for anyone that does know a True Name to ever admit or use them unless they have no other choice.
It's a common folktale throughout history, that of a mage showing off the knowledge of True Name, only to be hunted down at his weakest and tortured for the name that he had spent all his life learning. Either dying and taking its knowledge to the grave, or offering the name in hopes of being spared, or parts of the True Name, and widespread chaos and panic flowing forth in the aftermath.
Throughout all of these stories, one method also holds true. By making a contract with a powerful entity, the individual is offered the True Name of an aspect of reality that they rule over. In exchange for this great power, they then owe the entity some tremendous debt, and what it is has varied with the times and the entity in question.
Wielding a Name
All things, in reality, hold True Names, and this includes all aspects of nature. Something as simple as 'grass', as potent as 'fire', or as overwhelming as 'death'. As such when an individual learns the True Name of anything, they hold complete and utter dominance over it, not through their magic, but the very laws of reality. By simply knowing the true name, they are able to manipulate, create, and control whatever it is and empower it without any limits. It also renders that aspect completely powerless against them.
For instance, if someone knew the True Name of 'fire', if a mage released a fireball at them then without even a wave of their hand the fireball would not only be dispersed but would be under the complete control of the individual that knew the flames True Name.
For this reason, Mages consider the knowledge of True Names unequal in value. If it means learning the True Name of anything, even something as seemingly useless as say 'Grass', they would trade everything they own for it quite often as even something as seemingly weak as this, in the hands of even an amateur mage, would make them one of the most powerful plant-based magic users in existence to infinity spawn, create, and manipulate grass anywhere they wished at any time of any quantity. Even shifting its status to different "types" of grass to change its density or makeup depending on the mages knowledge of plants, chemistry, and alchemy.
It is in many ways, considered the 'purest form' of magic as it bends a small part of reality itself to your will. The only way to counter the knowledge of a True Name, is to know the name as well in which case it comes down to superior skill at controlling the magic instead.
Tiers of Reality
All True Names have a hierarchy of power in a way, that comes down to its parental aspect. In the way of the previous example, if a mage knows the True Name of 'grass' and goes against another user who knows the True Name of 'plants', then their own control is not only nullified but they are overwhelmed by the hierarchy of the other user.
In this way, True Names are not a vocabulary that one studies and slowly obtains more names. Instead the so-called “name” that one knows slowly evolves as they acquire more knowledge of reality around them. For instance, if the mage that knew 'grass' learned the word for 'tree', they would not then know two words, but instead, the word itself would evolve into that of 'forest' within their mind encompassing the ideas of both grass and tree together.
Similarly, that mage may then learn the True Names for 'flowers' and 'moss', perhaps even 'mushroom', and so forth and have that knowledge evolve even further into the True Name of 'plants' and match that user that completely bested him previously in the understanding of True Names. The higher up this hierarchy one goes, the more abstract the study becomes. Evolving from 'Plants' for instance would need to become something more, such as one of the base elements of existence such as 'Nature', in which case a mage would need to study and learn several more words based on water and earth such as 'spring', 'river', 'rock', and more.