During the Scattering, Angron was thrown to a "civilized" human world far from Terra. He plummeted into the icy mountains of that planet with an echoing crash, and not long after a trader found him, and a scene of carnage.
Surrounding the unconscious young Primarch were strewn the bodies of dozens of Biel-tan Craftworld Eldar. It is said that one of their Farseers saw the great bloodshed that Angron would cause and had tried unsuccessfully to stop him.
The trader clasped Angron in chains and brought him to one of the great cities of the planet. There, he was forced into the techno-gladiatorial arenas, where he proved to be undefeatable and a fan-favorite. Several times he tried to lead his fellow gladiators in revolt, and each time failed.
Yet once, he succeeded. He led his rag-tag army into the mountains of the planet, where he lived for several years. The civilized cities sent armies to destroy Angron, but they were all obliterated. Nonetheless, the issue was never in doubt. His forces had little to eat in the barren mountains, and were exhausted from the constant battling.
His fate seemed sealed when seven well equipped armies surrounded Angron and his starving forces. Just as the battle was about to begin, the Emperor's Fleet arrived in orbit over the planet. The Emperor teleported directly to Angron's point of deployment with a few trusted Adeptus Custodes.
The Emperor promised Angron a legion made in his image, limitless power, and life-times spent perfecting the Art of Conquest. But, to his surprise, Angron refused. He chose instead to die amongst his comrades while fighting his oppressors. Reluctantly, the Emperor returned to his flagship above.
Yet just as the battle was about to begin, the Emperor teleported Angron against his will back up to the fleet. He could only watch in anguish as those he regarded as his brothers and comrades were quickly annihilated.
Nonetheless, with nowhere else to turn to, Angron took full control over his legion. During the course of the Great Crusade, he reaped many victories, although some criticized the extreme and bloodthirsty tactics he used to ensure the destruction of his opponents.
The Emperor himself criticized Angron for the changes he made to his recruits. Like all other gladiators on his homeworld, Angron had received special nerve-implants which tremendously heightened his aggressiveness, but also had the side effect of uncontrollable rages outside of battle. Angron ordered his Techmarines to duplicate the technology and that all recruits undergo the process that would turn them into aggressive and fearless warriors. Despite the obvious advantages, the Emperor was displeased, and ordered him to stop. Angron paid him no heed and continued the unsavory practice secretly.
This led to the World Eaters being criticized for their general bloodlust and barbarity by their fellow Legionnaires. They were known for blood rituals when not in combat, and competed with each other for the number of enemy heads they could take in battle.
The Emperor dispatched Horus to bring Angron back in line - a fatal error, as Horus was already corrupted by the forces of Chaos. A master psychologist, he told Angron exactly what he wanted to hear: that the Emperor was a weakling devoid of honor and that there was a place for him in a new order, along with revenge against the brother Primarchs that had criticized his legion. Angron needed no further convincing, throwing his legion on Horus' side when the Horus Heresy broke out.
Already savage and brutal, the World Eaters quickly found a new master in the form of the Blood God Khorne, eternally demanding sacrifice and skulls. Angron led his legion into their worship, his form mutated to a twisted red monstrosity wielding a glowing runesword. Following their arrival in the Eye of Terror, Khorne elevated Angron to the role of Daemon Prince and gave him command of a Daemon world.
The most notable action Angron took after the Heresy was the First War for Armageddon where he and his demonic entourage were cast back to the Warp by an elite force of a hundred Grey Knight Terminators. He has not taken place in any major actions following this