10th Black Crusade

The 10th Black Crusade, is also known as the Conflict of Helica, and was a joint Chaos campaign carried out by Abaddon the Despoiler and the Daemon Primarch Perturabo. Launched 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, Thracian Primaris, the Iron Warriors focused their own assault against the loyalist Iron Hands Chapter to fulfil an old blood debt. In a series of bitter sieges, the Iron Warriors burn the Medusa System's worlds and push the Iron Hands to the brink of destruction. Only when the Loyalist Chapter receives heavy reinforcements do Abaddon and the Iron Warriors retreat, taking with them valuable information about Medusa's defences.

History
In the Tenth 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 of a chain of enslaved cogitators, allowed Abaddon's forces to steal past the layered defences of the Cadian Gate and into the Helica 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, home world of the Iron Hands Space Marine Chapter. From the Vengeful Spirit he watched as the Iron Warriors laid siege to the loyalist Chapter. In a protracted war of attrition, the Iron Warriors systematically burnt 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 allies could arrive, Abaddon and his Iron Warriors retreated, leaving hundreds of dead loyalist Space Marines in their wake. Though Medusa still stood, Abaddon had learned much about its defences and the Iron Hands themselves, both of which he would put to good use later.