Hi everyone!
Some days ago, I made a thread: I was seeking some The Pyre background, and someone told me that there weren't any background for them... and why about creating my own background?
So, taking some histories from some places (some official, some unofficial) here you are (sorry for the bad english):

THE PYRE CHAOS SPACE MARINES
When Khârn The Betrayer got crazy on Skalathrax and he killed all humans and Emperor's Childrens who were there (more info in
CSM codex), most of them were killed by his chain-axe. For making his victims to go out of their buildings, he used a flamethrower: he burned everything, and lots of people died, burned alive. In this bloody and flaming party, Khorne ate lots of lots of souls, but not all of them...
The pain and fear of those who died by fire created a new entity in the Warp. This daemon feed on the souls of these poor victims of fire, and become alive. His father: Khorne. His godfather: Khârn. His name: Kaathler.
Kaathler had a hard begginig. In the real space, not much people were often killed with fire, so it had a little power. He needed to hide, because if not, rival creatures would devour him, but because of its weakness, he could hide from Inquisition and Eldar. Finally, the day of his uprising came. When multiple races engaged the others on Armageddon, lots of daemons surrounded it, having fun with that feast of souls. Kaathler was there, and when the equatorial rainforests were set on flames, and uncontable soldiers died, burned alive, he got enough power to influence on the real world, to start his own fires, and to start his uprising: Kaathler was becoming a Chaos God.
He wanted to have his own cultists and a flaming homeworld for them, so he give his power to a human, the chaman of a distant Imperial world. When the Imperium lost contact with it, and the Imperial Guard went there, they saw that the whole world was in flames, like a star: the imprudent, young chaos daemon used too much power. The imperial commander declared it Perdita Diabolus, and made an Exterminatus on the planet. Whem the world and its few unhabbitants were destroyed, Kaathler howled, and all phsyckers on the galaxy known that a daemon did an important step to becoming a Chaos God.
In the next centuries, Kaathler annexed
CSM to his ranks, most of them from the World Eaters legion, those who enjoy watching their victims burned alive, to taking their heads off (only those who survived to the rage of Khârn, who was really angry with those who wanted to betray him and Khorne). This
CSM formed The Pyre, a traitor legion consacrated to Kaathler, Fire’s God. They painted their armours with orange and black, and put on them flaming skulls symbols.
The Pyre
CSM took a Nurgle world, and Kaathler’s sorcerers made on it a gargantuan volcan, and renamed it “Infernus” (Kaathler likes to use some magic, unlike Khorne). Because of this, Nurgle got angry with the young god. This is a serious problem to Kaathler: he’s not strong enough to fight a chaos god’s cultists, but he will…
The Pyre
CSM also travel with Red Corsairs, doing bloody, flaming raids against their enemies, for giving more power to their god.
Ok, that’s all. Give me your opinion. Thank you!