The Death Guard betrayed the Emperor and now serve Nurgle and share his desire to spread his contagion and corruption across the universe.
Via our good friends at Lexicanum
At the beginning of the Horus Heresy, many Death Guard who remained loyal to the Emperor were massacred on Isstvan III by their fellow Space Marines, including Captain Ullis Temeter. Roughly a third of the Legion was still loyal to the Emperor. Shortly after, they battled Imperial forces in the Drop Site Massacre.
During the Horus Heresy, the Death Guard joined Warmaster Horus in many battles and raids on the Imperium. The Lord of Death split his fleet, commanding one himself and Calas Typhon the other. Mortarion’s smaller fleet led a failed attempt on Prospero to convince Jaghatai Khan and the White Scars to join with them, and the Mortarion found himself in combat with the Great Khan. After the White Scars managed to abandon the Death Guard fleet, Mortarion had his Legion embark on a spiteful purge of the Prospero System. Mortarion then fought alongside Horus in the Battle of Dwell and Battle of Molech before rejoining Typhon’s main fleet, which had been waging a campaign of misdirection and misery against the Dark Angels since the Battle of Perditus. Later, Horus himself tasked Mortarion with finding and destroying the White Scars. Eager to settle the score with the Great Khan, the Death Guard and Emperor’s Children allies under Eidolon cornered the Scars at the Dark Glass, but failed to destroy them in the Battle of Catallus. Angered, Mortarion realized that his divided legion was hampering his war effort and ordered Eidolon to find Typhon and his splinter fleet.
When the Death Guard’s fleet embarked for Terra, Typhon killed the Navigators, whom he alleged remained loyal to the Emperor, and assured his Primarch that he could lead the fleet to Terra without their help. Instead, he led them into a trap – becalming the Death Guard fleet in the warp, adrift, helpless and at the mercy of Chaos. Then came the Destroyer Plague and the Death Guard were struck down, but Typhon received his reward from “Grandfather Nurgle” and he absorbed the full power of the plague. His body became home to the flies of Nurgle, his armor a hive of pestilence. He then became Typhus, Herald of Nurgle and the Host of the Destroyer Hive. A number of Death Guard Marines, and one Luna Wolf who renounced his Sons of Horus status, led by Battle-Captain of the 7th Company Nathaniel Garro, remained loyal to the Emperor. They formed part of the crew of the Eisenstein, a frigate which ran the Traitor blockade in the Isstvan system in order to bring news of Horus’ descent into Chaos to the Emperor on Terra.
Despite Garro’s efforts, the forces of Horus besieged Terra and the Imperial Palace itself. After a breach in the Palace defensive wall was forced by Titans of the Legio Mortis Titan Legion, the Traitor Legions, including the Death Guard, poured into the breach only to be met by loyalist forces led by the Primarchs Rogal Dorn and Sanguinius. It is recorded that Mortarion personally led his pustulent Plague Marines into the thickest fighting that day. After Horus’ defeat, Mortarion led his Death Guard in a campaign of destruction over a score of planets, until finally retreating into the Eye of Terror. Here he received Nurgle’s ultimate reward and became a full-fledged Daemon Prince, ruling over one of Nurgle’s greatest Plague Worlds in the Eye of Terror. Mortarion sends out fleets of Plague Ships into the Warp to carry their contagions throughout the galaxy. Concerned himself with matters of the Warp more and more, Mortarion has periodically returned to lead his Legion but in his absence it has largely splintered into many smaller warbands.
However though it was factionalized, the Death Guard never fully disintegrated as a cohesive force. Their fragments continued to fight under a singular purpose, and never resorted to the civil in-fighting of many other Traitor Legions. At the end of the 41st Millennium, Mortarion senses his Brother Primarch’s rebirth and reasserted direct control over the Death Guard once more. The Death Guard launched a major assault on Ultramar in what became known as the Plague Wars.