The Soul of Cinder is such a great concept bc it is literally the result of what happens if you refuse to let something die. It's something that needs to die, the whole world practically BEGS for it, but it still fights on bc of all the lords of the past who chose to keep it alive anyway.
Like, imagine a garden that is never trimmed but ever watered. It needs to be pruned, it needs to be restrained, it may even need to die in the winter so the farmer can have another go, but instead the farmer keeps watering it, keeps feeding it, until it grows out of proportion, leaves and vines tangling up fences and spilling out into the yard around it. In accepting the gift of life given to it, it has become deadly to the things around it, as it chokes out the life from other plants and ruins the natural order of things. Yet it can no longer be simply pruned in an afternoon now, no, now it's become a whole landscaping job (and as somebody who worked landscaping for a summer job, it's a nightmare). It took the life you gave, and it'll be damned if it gives that life back without a fight.
And that's exactly what we see with the soul of cinder, something that has to be killed, but it's clinging on to the gift of life with everything it's got. It's the indomitable human spirit in its most pure, terrifying form. Pulling out everything that it's ever been given in an effort to keep holding on to life.
Yet life must give way to death; the rain must drive away the sun, and the grass must wither and fade away.