Warning, lots of fluff. The actual usage is in the tldr.
Crown of ash
To say it was a fallen star would imply it was pure, or holy, but stars are grown in the void itself. Divine perhaps, but holy was the wrong direction.
When our men first layed eyes on the meteor, they knew what it was. A group of Moskurg out riders had taken it before we arrived and they were making a break for friendly territory. We rode them down. Slaughtered them. When the last few threw down their arms and begged for mercy, our captain had their heads. But first he had their eyes, and their hands. There, as the blood flowed around them, the ground littered with corpses, there it sat. Untouched, clean on a bed of white cloth, it's color a dull dark red.
On the journey back, the captain who had found it had his tent burn down, with him inside it. They found the meteor unmarked in the center of the ashes. Cool to the touch, even as the coals glowed beside it.
They took it to the highest wizard in the land, who sought to study it.
When men first came to understand magic, they described it as another plane, sitting just above this one. Just out of sight, out of reach. When a mage casts a spell, he gathers his mental strength and punctures this barrier between the planes, and his spell is woven as he shapes the energy as it pours through into our world. Such was the basis for all known magic.
The meteorite was something different. The high wizard ordered a crown to be forged from it, by the best Smith of the land. It took three days of concentrated magical flames to heat it far enough to work with. When the Smith first struck the metal, a dark red blood began to flow forth from it. When it was measured, it was found to be 13 cups worth. One cup for each moskurg prisoner, killed and tourtered before it.
Magic comes from a higher plain, but this was something else. It was like magic, but it's plane sat below ours. It's energy's were foul and incomparable, alien and foreign to all we knew. Any who had tried to pierce this barrier before had found that it was their minds that were punctured and shattered. But this was something different. It's connection to this force, it was almost like something from the other side, trying to force its way through. The lower barrier was weak here, but it needed someone to reach out to it from this side.
When the crown was finished, it's color was a dark black. More than black, it was a pure absence. Small drops of blood still dripped occasionally from the tips.
The high wizard called 5 of his greatest and most trusted pupils into his chamber, and 60 of the high guards, clad in resplendent plate male and welding swords of shinning steel.
In the middle of a circle there, with his whole host arrayed, he took the six pointed crown, and placed it upon his head.
He cried out in pain and terror as his eyes melted and ran from their sockets. The crown sunk into his head, until the sound of metal scraping on bone was heard. His students cried out in fear, and they pulled forth amulets of anti magic, but the crystals exploded with such force that all his students were torn apart.
The guards cried out and drew their swords. They made as if to advance on the wizard, but he waved his hand and they each feel to the ground, contorted in pain and fear as each changed into a monster defined by their own worst sins.
The ashen king descended now with his host, and killed all the rest of his castle, magician and soldier and mage hunter alike.
There must have been something left of the old wizard in him, for he turned to the front then. He ignored his former allies, so long as they ignored him.
Now he fights at the front, unstoppable. His fellows shiver, thankful he is on their side, and quietly wonder what he shall do when the war is done...
Some say he had tapped the powers of hell itself, but they are wrong. This is not hell. This is the thing that killed hell, and rules in its stead.
TL, DR:
Essentially, it is using mind magic to flip the plane of magic the user draws from. It's very powerful, but somewhat uncontrollable. (The demon thing was just fluff)
All the users spells are inverted, more powerful versions of the original.
Ex: a regular wizard casts a fireball, and it's a yellow ball of flame that explodes outward.
The ashen king calls a fireball, and it's and enormous ball of grey flames, that implodes, sucking all nearby thing into it. The flames are impossible to put out, and release noxious fumes.