Quotes often vary in meaning to the individual but my take would be:
A true warriors will to fight implies that solely when it is time to place what they consist of ~ the sword or life line all the skills they have developed until now it is only there and then when you truly find what they are; what they heart is. It is precisely to say that the sword is a reflection of their entire life they have as warriors poured in all their effort into refining their techniques or becoming better at wielding this sword. As such what they are is one with the sword, if they are without it you are not really seeing their true self. Is what I would interpret this as. However the part of ‘until their sword reveals it’ indicates the actual act of displaying their everything with the sword. “All is an act until the sword is unsheathe; there are no lies in the dedication poured into your blade”. Is kind of what I would say provides similar meaning, keeping in mind that sword can refer to yourself as a sword or your actions, not every warrior fights with a sword but this would still apply to them (as an example).
This can be described as an end of sorts. In short the slow burning refers to a gradual end out of the lack of the darkness (normally such quotes indicate abundance of darkness rather than lack of but this seems to indicate this). A darkness which is not filled with greed but greatly so with appreciation. This darkness then eradicates itself from within with the message that it would not return. Such a thing causes great grief and loss but also this loss paints pictures in the form of pain, pictures that display the entirety perhaps a beauty.
Now I actually think what the quote was trying to say was more along the lines of :
because I no longer have made attempt to contain it my darkness inside is running loose. As such this same darkness is devouring me on the inside all for the promise of not returning to the way things used to be, the pain I feel paints pictures that but attempt to display the entirety.
Pictures say more than a 1000 words-> they can at times tell an entire story , when you apply this to the above it adds more weight to it.
The reason I say actually think after I have given my previous explanation is because this ‘feels’ more natural to the quotes but its not a direct output as much as my previous statement was. The second is a lot more holistic.
Which all overall provides more reason as to why quotes are interpreted differently.
Quotes often do not mean literal things, they are more akin to playing video games to learn physics, you are interested in properties which are similar , you are free in what you do , you deduce through associations,abstraction,even overlapping regions and gaps even at times your own bias.