At the dawn of the 37th Millennium Antecanis was a teeming Hive World that supplied labour to the nearby Imperial Navy dockyards at Cancephalus. Knowing that the Imperial Navy docks were heavily defended, Abaddon chose to strike at its most vulnerable point - the world from which the workers came. Without the supply of millions of workers, Cancephalus's dockyards would fall silent.
Making various pacts with other Chaos forces such as the World Eaters, Word Bearers, The Purge, Apostles of MinthRas, and the Daemon Prince Dhar'leth, Abaddon and his allies fell upon Antecanis. Abaddon's Battle Barges and Grand Cruisers overwhelmed the orbital defences. After seven days, fusion bombs and vortex torpedoes rained down from orbit onto the Antencanis Hives. As millions died, dropships poured onto the surface and disgorged hundreds of ruthless Chaos Space Marines.
Abaddon's lieutenants and allies had divided the spoils of war for themselves and set about attacking their targets. Abaddon focused upon Monarchive, the seat of the Imperial Governor, with Dhar'leth as his vanguard. Darh'leth's assault was relentless; for sixteen days the Daemon Prince and his Daemonic hordes supported by Black Legionnaires burned and butchered their way through the lower levels of the Hive. Refugees fled in the tens of millions, but lines of slaves hundreds of kilometers long were captured and trudged back into the Black Legion's landers.
When the Chaos forces attacked, Astropaths had broadcast a warning and called for aid. Space Marines of the Silver Skulls Chapter were the first to respond. In daring hit-and-run attacks they harried the Chaos fleet and robbed the heretic forces on the surface of orbital support as the Departmento Munitorum raised regiments for an all-out war. Abaddon knew that once the limitless numbers of the Imperium were brought to bear, Antecanis would become a cauldron of battle from which there was no escape. Thus he resolved to swiftly end the siege of Monarchive.
Abaddon led the final assault on the capital with his bodyguard of Black Legion Chaos Terminators. They slaughtered Adeptus Arbites and soldiers of the Planetary Defense Forces in a two day massacre. With Monarchive now under his control, Abaddon had his forces kill or capture as much as the Hive's population as possible. The old and the sick were executed while others were transported to slave ships waiting in orbit.
When Abaddon received word that the first Imperial Guard troops had arrived in orbit, he ordered his lieutenants to leave Antecanis at once. Giving no warning to his allies, he used them as pawns, knowing they would fight and delay the Imperial pursuit. With his ships crammed full of loot and slaves, Abaddon departed Antecanis and broke through the Silver Skulls fleet, dropping a dozen Melta Torpedoes onto the ruins of Monarchive as a last contemptuous gesture.
Imperial Guard regiments from Cadia, Lostak, and Second Hubris arrived in the following weeks and months, and the Chaos forces left behind fought grimly. Some escaped the tightening Imperial noose while others fought to the last in bloody defiance. As Abaddon planned, the Cancephalus shipyards gradually stopped in construction and within a year was unable to refit any Imperial Navy ships, leaving Abaddon's fleet able to ravage the sector at will. The Imperial Guard and abandoned Chaos Space Marines continued to battle for possession of Antecanis for seventeen years. The world has never recovered from these battles and has been reduced to ruins and ash wastelands.
Codex: Chaos Space Marines (6th Edition) describes the Antecanis Massacre and 9th Black Crusade as occurring in 573.M38.