Cleansing of Ariggata
Towards the end of the Great Crusade, Horus assembled three Legions to take back the technologically advanced planet Ariggata from the separatists who controlled it; the The Sons of Horus, World Eaters, and Ultramarines. Most of the planet was quickly conquered, except for the massive fortress within which most of the leaders of the planet cowered. Eager to be on his way, Horus commanded Angron to take back the citadel and kill only the leaders.
Eagerly, Angron led the assault. However the fortress was heavily defended and the casualties were horrendous, a dozen World Eaters falling for a meter of land. Eventually, a ramp of corpses led up to a single breach in the wall, and the Marines plunged in. Filled with rage over their fallen brothers, they were merciless.
By the time the Ultramarines arrived, the battle was all but over. The inside of the fortress was filled with the dismembered and mangled corpses of the defenders, for not one soul had been spared the vengeful fury of the World Eaters. It was a slaughter.