Codex Adeptus Astartes: Space Marines (2017), p29 — M41 The Time of Ending

The Plagues of Ultramar

To the galactic north of Ultramar, the followers of the Chaos God Nurgle establish dominion in the Scourge Stars. From this hive of corruption, armies of Daemons pour forth, accompanied by the traitorous Death Guard Legion along with renegades and cultists beyond count. Three loathsome spearheads push into Ultramar, attacking along a hundred fronts and bringing with them unnatural pestilence. The defenders of Ultramar fight bravely, but lose ground. Ultramarines from the Ultima Founding arrive to reinforce their brethren, but they can only slow the attackers.

Guilliman Returns to Ultramar

All across Ultramar, the Imperial defenders are depleted in dozens of ground campaigns, while a Plague Fleet systematically destroys the realm's defence fleet and star fortresses. Guilliman returns from the Indomitus Crusade, and his deft and defensive manoeuvres buy time to launch the Spear of Espandor counterattack. The combined plague armies are eventually fought to a standstill amongst the ruins of Iax, before the Death Guard Daemon Primarch Mortarion escapes under cover of a virus bomb.