The Fire People
The Weapon: The Fire People (African culture) have a weapon that is sacred and very distructive. It is the ability to wield a fire which penetrates the very soul and destroys any evil parts that are in a soul. If there is none, then the flame will not inflict harm. If the whole soul is evil then the flame will take the life of the individual because there is little or no good left in them. This is why they were placed somewhere hidden so that this weapon could not become abused.
The People: These people were going to be completely made out of fire, however, there is a better solution I believe and here it is: The people will not be made of fire but will have a translucent flame-ora constantly hovering and moving all around their body. It will look somewhat like a spirit but it will appear to be a burning flame either blue or red or something. The history of such an existance is that they decided to control the level of purity in the population of the people so they started to use this flame to purify the people. And it somehow stuck. (more explanation to follow).
The Flame and The Sword: The Sword will be called “The Sun” because of its part to play in the “dawning of time” when the world will be on its last stretch of existance. Like the Fire People, the flame will hover and glow around the blade and maybe the hilt of the sword. It is also “The Sun” because it always has a glowing flame around it.
Now, this flame will be powerful because the sword, as a wielder of the flame, will penitrate the soul far more than the flesh. And as “The Dark One Lurking” is completely evil, the blows will and can be fatil.
Their Final Role: When Cul colides with the Old World the Fire People will attack with this Flame that will purge the land, purifying the ground as well as the people and kill all people that are evil. But, their flame cannot defeat the dark one. He is too evil. He needs a flame more powerful than the one the fire people can produce. That flame comes from the sword.
“the dark one lurking turns to ash” - the flame that is used to defeat “The Dark One Lurking” will purge him from all of his evil and defeat him turning him to ash by way of Flame. This has religious conotation which should be easy to understand.