The 10th Black Crusade, also known as the Conflict of Helica, was a joint campaign of the Forces of Chaos that was led by Abaddon the Despoiler and the Daemon Primarch Perturabo. Launched out of the Eye of Terror in 001.M39, both the Black Legion and the Iron Warriors Traitor Legions struck against the Helica Sector.
While the Black Legion attacked the capital world of the sector, the Hive World of Thracian Primaris, the Iron Warriors focused their own assault against Medusa, the homeworld of the Loyalist Iron Hands Chapter, to fulfil an old blood debt. In a series of bitter sieges, the Iron Warriors assaulted the Medusa System's worlds and pushed the Iron Hands to the brink of destruction.
Only when the Loyalist Chapter received heavy Imperial reinforcements did Abaddon and the Iron Warriors retreat back to the Eye of Terror, taking with them valuable information about the world of Medusa and its defences to be used at a later and more opportune time.
In the 10th Black Crusade Abaddon forged a dark alliance between the Black Legion and the Iron Warriors, making a deal with the daemonic Primarch Perturabo. In exchange for Abaddon agreeing to settle a debt of vengeance against the Iron Hands, Perturabo opened an ancient Warp route out of the Eye of Terror for the Despoiler's Black Fleet.
The passage, calculated over several agonizing months by the tortured Machine Spirits (artificial intelligences) of a chain of enslaved Cogitators, allowed Abaddon's forces to steal past the layered defences of the Cadian Gate and into the Helica Sub-sector of the Scarus Sector, spinward of the Eye of Terror. Once again, Abaddon employed a carefully planned feint, sending the Black Legion fleet against Thracian Primaris, the capital world of Helica, while he attacked elsewhere. The Despoiler also scattered his warlords across the sector and, on scores of planets, Traitor vessels thundered down form the skies to kill and maim Imperial citizens.
While his warlords indulged themselves in the slaughter of Imperial worlds, Abaddon travelled with the Iron Warriors to the Medusa System, the volcanic homeworld of the Iron Hands Space Marine Chapter. From the ancient flagship Vengeful Spirit Abaddon watched as the Iron Warriors laid siege to the Loyalist Chapter.
In a protracted war of attrition, the Iron Warriors systematically assaulted the outer worlds of the Medusa System, tearing down the Loyalist defences one by one. In the burning light of their ruined fortifications, the Iron Hands struggled to hold back the Traitors, buying time for their support companies to arrive.
Before the bulk of the Iron Hands and their Imperial allies could arrive, Abaddon and his Iron Warriors retreated, leaving hundreds of dead Loyalist Space Marines in their wake. Though Medusa still stood as an Imperial bulwark, Abaddon had learned much about its defences and the Iron Hands themselves, both of which he would put to good use later.
- Black Legion: A Codex Chaos Space Marines Supplements (Digital Edition), pp. 39-40
- Liber Chaotica, pg. 85