Deathshroud
"The Pale Harvestmen; the Scythes of Nurgle; the Eyes of Mortarion. The elite warriors of the Deathshroud go by many names, and every one is redolent with a miasma of fear and menace. Swollen with unnatural power, the Deathshroud tower over their enemies. Rusted gauntlets and squirming tentacles clutch huge battle scythes known as manreapers, cursed weapons that slice heads from shoulders and limbs from torsos with every swing. Clouds of plague flies boil around the Deathshroud, while vile smog spills from vents in their armour to choke and blind their foes." ~Games Workshop
The Deathshroud are the original and current bodyguards of Daemon Primarch and current Edgelord Supreme, Mortarion. Other than appearances, little has technically changed from their days during the Great Crusade, of course now they have more of a Nurglite twist.
Great Crusade & Horus Heresy
In the beginning of things, the Deathshroud started as Mortarion's Honour Guard. Great secrecy surrounded them; they were chosen from the rank-and-file of the Death Guard by their Primarch personally for their skill at arms, fearlessness and proven endurance; often being the sole survivor still standing when the rest of their squad has perished. As Morty wasn't one of the most social of Primarchs, his introverted social skills seem to spread to his sons as the Deathshroud's identities were basically 'unpersoned', since, after being inducted into the Deathshroud, they never spoke, never removed their helmets even in death, and were officially listed as being killed in action which is very convenient after the whole Nurgle fiasco. (to be fair, Mortarion was one of the few Primarchs who actually valued his bodyguards instead of seeing them as mere necessary pomp or a hindrance.)
Deathshroud bodyguards were clad in highly artificed Terminator armour that gave them a tall and barrel-chested appearance and they were armed with hand flamers with chem-munitions and like their master, they wielded large power scythes known as Manreapers. Although given the Deathguard's overall lack of care for their own equipment, their prestigious equipment was left to rot and rust away, giving them an appearance of a warrior that had been through countless battles and war. They never strayed more than 49 paces from their Primarch. A trait they would continue right into the 41st Millennium (And how convenient for Nurgle too, since the square root of 49 is 7, Papa Nurgle's favorite number, coincidence? Who knows?).
During the event of Horus' potty tantrum, the Deathshroud along with their Primarch succumbed to Papa Nurgle's plagues like pussies and became physically twisted by Nurgle's gifts after they accepted their allegiance to the Plague Father.
The 41st Millennium
As a Daemon Prince of Nurgle, Mortarion was still accompanied by his Deathshroud ten thousand years later, notably when he confronted the Grey Knights at the Battle of Kornovin.
On top of that, the Deathshroud serve as Mortarion's representatives. While they still often fight by his side they also form the honour guard for Chaos Lords, Plague Surgeons, and other champions of Chaos which have garnered Mortarion's favor. Their presence is a mixed blessing at best. The Deathshroud will fight with all their skill and strength in support of their assigned champion, and prove to be an undeniable asset in battle. Yet all the while the Deathshroud are judging their charge upon their master’s behalf. If the champion is deemed successful, the Deathshroud depart at the battle’s end, leaving as silently as they came. Should the one they guard fails in his duties, however, the judgement of Mortarion is swift, deadly and utterly inescapable.