Selfless State

The Selfless State (の) or the Realm of the Highest/Domain of Supremacy is a mysterious state of being associated with the Transparent World and is implied to be the zenith of physical combat.

Description
The Selfless State is a supernatural, esoteric and mostly mysterious state of being in which one completely erases their presence from the battlefield, completely getting rid of their fighting spirit/will to fight, bloodlust, anger, hatred, malevolence, drive and animosity. This is extremely effective against most high-level combatants in the series who rely on various forms of extrasensory perception to detect malice, aura or fighting spirit.

Little is known about the existence of the Selfless State, but from what Akaza has remarked about it, it can be surmised that the Selfless State is the highest state of physical combat one can ever achieve, though it does not enhance one's own combat prowess. Once in this state, the user's behavior becomes incredibly tranquil and focused, often being compared to that of a plant.

The Selfless State and Transparent World are also closely associated to one another to the point that unlocking either one can often lead to unlocking the other.

Unlocking
Not much is known about how a combatant can access the Selfless State, but based on how Tanjiro Kamado learnt it in the midst of battle, one must be able to close of all of their senses and clear their mind, focusing on a particular moment so much that they reach a stage where they are completely detached from themselves. However, it is still unclear whether one has to be marked or possess mastery of Sun Breathing in order to achieve this state, though it is implied to be the case.

Trivia

 * The Selfless State is also often referred to as the State of Anatta, a state with striking similarities to the Selfless State which works as a state of nothingness where one is so focused on the fight that they become completely detached from everything else and can engage in combat without wasteful thoughts and movements. The nothingness one experiences in this state is akin to plants on a battlefield, where opponents are always on the constant lookout for moving targets/animals while presenceless plants in the background are usually completely ignored. Thus, to be in the State of Anatta is to "be" the plants on the battlefield, to mesh oneself in the background so much until their opponents don't notice them until its too late.