From the announcement:
The Death Guard are the Traitor Legion most favoured by Nurgle. Utterly devoted to spreading the Plague God’s hideous diseases across realspace, they are living plague vectors whose bloated bodies and rusted war engines boast grotesque resilience and firepower. They are a walking pestilence, a living plague of destruction and horror. Their heavy tread heralds the end of hope, and the damnation of all who oppose them. Their touch withers all, and spreads seething corruption that swallows worlds. They are the sons of Mortarion, the chosen of Nurgle; fear them , and despair.
Codex: Death Guard contains a wealth of background and rules – the definitive book for Death Guard collectors.
For ten thousand years Mortarion, Lord of the Death Guard, has crushed his enemies upon the field of battle. Surrounded by chittering Daemon mites, droning flies and noxious plague vapours, Mortarion swoops into battle on vast, creaking wings. the thudding beat of these foul pinions fills the enemy with crawling dread, even as they waft the reek of death across their lines. To even stand in the presence of the Death Lord is lethal, opponents choking and collapsing as they succumb to the myriad plagues that churn the air around him.