The "Vessel Known as Destiny" (運命という名の器 Unmei to iu Na no Utsuwa) is the concept of thinking of the fluctuations in size of the Power of Existence of an individual of this world due to the present, past, and future that they possess, three-dimensionally. It is equivalent to the time integral of Power of Existence. In other words, it is all of an individual's potential or possibilities and their influence on this world and other existences, and it could be said that a human's "Vessel Known as Destiny" is "everything that makes them human".
Background[]
The "Vessel Known as Destiny" includes an individual's influence not only when they were alive, but also their influence before they were born and after their death. It is not decided solely by that individual; their achievements and the achievements of their descendants can lead to it spreading.
The amount of Power of Existence an existence of this world has at the present time always fluctuates with time. The size of an individual's Power of Existence at the present time and the size of their "Vessel Known as Destiny" isn't necessarily proportional, and it is difficult to actually measure it. Fundamentally, it becomes smaller before birth and after death, but for royalty, just being born carries a lot of meaning and thus they would have a lot of Power of Existence before they were born (an example would be Khamsin Nbh'w), and Shana, an abandoned child, had a large one, so there are a lot of exceptions.
The size of power of Flame Haze, who make the vacuum where they have lost their "Vessel Known as Destiny" into their Vessel, and the amount of Power of Existence first possessed by humans who directly became Mystes (like Tenmoku Ikko and "Eternal Lover" Johann), are related to the "Vessel Known as Destiny".