The "Maggots of Despair" are a large sub-faction of the Death Guard Legion. It is believed that the Maggots of Despair are primarily composed of elements from the Death Guards 5th Great Company. They adhere to the combat doctrine's taught to them by their Daemon Prince Primarch Mortarion, and are fiercely loyal to him.

They were first encountered during Abaddons 13th crusade. Their infamous raids throughout the Agripinaa Sector were notoriously brutal.  Also known as the Tyrants of Yayor, the Maggots of Despair also participated in the massacres at Dentor and Malins Reach. Their last known sighting during the 13th crusade was the Deathguards unsuccessful attempt to incite an uprising in the capital hive city of Agripinaa Prime. Unconfirmed reports had sightings of Necrosius and the remnants of the Apostles of Contagion working closely with the Maggots of Despair during the failed uprising.

Soon after the 13th Crusade, the Maggots of Despair participated in the assault on the Secleris System.  The Secleris System was but one of many systems in the Aftermath Sector that was placed under siege by yet another Chaos Incursion.  During the campaign, the Maggots of Despair were not only responsible for the corruption and desecration of the former paradise world of Ignus,

"The forces of Nurgle were blasphemously triumphant in taking the planet Ignus. The former paradise world has been corrupted into a cruel and festering display of the Lord of Decay. The very waters pus and boil with a foul intent, the skies blackened as soot and filled with a pestilence so putrid it eats away all rational thought. The will to fight among the defenders has been lost; long have they fled from their doom, only to be encapsulated by the babbling insanity all around them. The faith of the Emperor has been extinguished on Ignus, perhaps even permanently." ...courtesy of the Aftermath Campaign

but they were also instrumental in the destruction of the Relic of Flamberg, securing of the Arcus Hold Station, the defeat of Inquisitor Fortez’s assault on Karsonis, and the successful defense of the Tabulis Rift.

At present, the Maggots of Despair have been sighted in the Vacuna System.  The planet Vacuna was once a thriving Hiveworld, but is currently being ravaged by the Waagh of Ork Warboss 'Eadsplitta.  It is unclear why Grandfather Nurgle has instructed Mortarion to send some of Deathguard there, but the Imperial relief forces en route to the besieged system are in for a more daunting task than what they bargained for.