|This article is about the Black Legion invasion; for the earlier Chaos invasion, see Invasion of Ultramar.
Shortly after the destruction of Cadia in the Thirteenth Black Crusade, Chaos Warmaster Abaddon the Despoiler recieved a disturbing prophecy from his Sorcerers, who informed him that the Tech-Priest Belisarius Cawl had a device of profound importance that could turn the tide of the war. The Sorcerers also informed Abaddon that Cawl was among the Imperial refugees from Cadia, and had been escorted into the Webway by the Eldar with Macragge as their destination. Hoping to beat to halt Cawl's plan, Abaddon sent a fleet of Chaos Space Marines to Ultramar.
The Imperial survivors of the 13th Black Crusade, led by Saint Celestine, Marshal Amalrich, and Inquisitor Greyfax, first emerged from the Webway along with their Eldar allies on the Shrine World of Laphis in the Macragge System. Despite the uneasy alliance between the Imperials and Eldar, necessity saw them fight together effectively and soon the area around the Webway Portal was secured from the Black Legion, which was taken by surprise. The newly arrived forces then moved on a besieged Ultramarines Fortress. After relieving the Ultramarine garrison within the Fortress from Heldrakes Saint Celestine successfully convinced the Astartes to allow them passage to Macragge itself as part of Cawl's plan. However all of the Ynnari besides Yvraine and the Visarch go back into the Webway, their mission accomplished for now.
Arriving at Macragge via Strike Cruiser, the Imperial forces find the capital of Ultramar under full-scale Chaos invasion. The Ultramar Defense Fleet is desperately engaging in an orbital battle against Black Legion, Iron Warriors, The Purge, and Night Lords. After fighting their way through the Chaos blockade, the Imperial forces land on Macragge and move on the Fortress of Hera, which is now besieged. At Hera they find Ultramarines Chapter Master Marneus Calgar and the other senior officers of the Chapter. Cawl reveals that he needs to see the comatose body of Primarch Roboute Guilliman, provoking outrage among the Ultramarines. However Cawl reveals that he is in fact over 10,000 years old and has been working on a way to bring back Guilliman on the orders of the Primarch himself shortly before his wounding by Fulgrim. Cawl reveals a new suit of Power Armour for Guilliman that can regenerate his wounds, as well as a new weapon for the Primarch. With Yvraine at his side, it is now possible for Cawl to fully restore Guilliman. With the backing of Librarian Tigurius the Ultramarines eventually relent and allow Cawl and his entourage into the Shrine of Guilliman.
As Cawl begins his work, the Black Legion makes one final desperate assault on the Shrine to try and prevent Guilliman's return. The Calgar, Saint Celestine, and Grey Knights 3rd Brotherhood led by Grand Master Voldus lead the defense and buy Cawl the time he needs. Ten Dreadclaws smash through the shrine, disgorging a hundred Black Legion Chaos Space Marines. Soon enough the Imperial defenders are all overwhelmed by this force, but as all seems loss Cawl and the others are successful in their work and the restored Roboute Guilliman enters the fray.
Face with the onslaught of the Primarch, the Chaos invaders are completely broken while the Imperial defenders are reinvigorated by his presence. Devastated by the Primarch's onslaught, the Black Legion is forced into a retreat. Enraged at the desecration of his homeworld Guilliman makes sure none of the traitorous Marines leave the Shrine alive.
In the aftermath of the battle Roboute Guilliman leads a general counterattack across Ultramar, devastating the Chaos invaders. With their mission now a failure, the remaining Chaos fleet flees Ultramar. Once at peace Guilliman adjusts to the reality of his father's Imperium 10,000 years later, now an empire of decay and superstition. After being fully briefed by his Ultramarines Guilliman declares that he will journey to Terra, beginning the Terran Crusade.