Nuceria

Nuceria was a planet in the Ultima Segmentum and the original homeworld of Angron. Had he landed absolutely anywhere else, the World Eaters may not have fallen to Chaos.

Nuceria was politically divided into multiple petty city-states and alliances, which constantly contested against each other for dominance. Within the city-states, various noble houses would constantly contest against each other for dominance. As you can imagine, this resulted in a poorly administered little crap hole, even by the relatively low standards of the Age of Strife. To keep the populace from revolting, the noble families used archaeotech known as the Butcher's Nails on slaves. The Nails dulled all other sensations in life and rewarded acts of aggression, turning the slaves into psychotic gladiators who would kill each other without a second thought. This in turn kept the populace distracted from how crappy their planet was.

Onto this shithole of a planet Angron was flung by the Chaos Gods. After killing some Eldar trying to stop his ascension, Angron was captured by the city-state of Desh'ea and made a slave. Angron managed to rally the gladiators into a rebel army, laying waste to every city they came across. Unfortunately, they were driven into mountains and awaited the Desh'eans to come and kill them. It was then that the Emperor of Mankind came to Nuceria and found his lost son, offering the command of the XII Legion Astartes, the War Hounds. Angron...told him to screw off, saying he would die with his brother and sister rebels, even killing one of the Adeptus Custodes with the Emperor, forcing him to retreat. Soon, the Desh'eans came, and Angron lead the charge...only to teleported onto the Adamant Resolve, flagship of the XII Legion. The Desh'eans thanked the Emperor for putting down the Gladiator-King's Rebellion, wiped out the rebels, and were promptly compliant. However, instead of making Nuceria the War Hounds homeworld, they planet was essentially forgotten as a backwater. As long as it paid the Imperial Tithe, the Imperium of Man essentially ignored Nuceria.

Well, a century later, Angron returned to Nuceria. His brother Lorgar Aurelain, Primarch of the Word Bearers, was forming a massive Warpstorm to cut off Ultramar from the Imperium as part of the Horus Heresy. Angron wasn't too thrilled about going back to Nuceria, as he had felt cheated of the death he felt he had earned, but when he learned that the Desh'eans had claimed he fled from the final battle that resulted in the defeat of his rebellion, Angron ordered the World Eaters and the Word Bearers to wipe out every living thing on the planet. By the eighth day, the Ultramarines, fresh from the battle of Calth, came calling. While Roboute Guilliman fought Lorgar, Angron intervened. But when Guilliman shattered the skull of one of Angron's fallen comrades, Angron went into a rage, and Lorgar used that rage, along with the suffering they had caused Ultramar, to turn Angron into a Daemon Prince of Khorne. Angron then proceeded to kill the last of the World Eaters Librarians, who had tried to kill Lorgar in a final attempt to keep their Primarch from falling, and forced the Ultramarines to retreat. Nuceria was left a dead world, as forgotten in death as it had been in life.