Codex: Chaos Space Marines (1999), p31 — The Death Guard

When Horus led his attack on the Emperor, the rebel Death Guard Legion was marooned in the warp where a mysterious contagion began to spread from ship to ship. The stinking pestilence bloated the gut, distended the flesh and turned its victims rotten from the inside. Eventually Mortarion became infected and in his delirium he called upon the powers of Chaos to aid the Space Marines. Mortarion's fevered ravings were answered by Nurgle and Mortarion became Nurgle's Champion.

The Death Guard, newly dedicated to Nurgle, arrived in time to take part in Horus' attack on earth and fought in all of the major battles of the campaign. As disease began to disfigure the Death Guard, their appearance changed into the disgusting form they still bear today and the once tall and noble Space Marines of the Death Guard became Plague Marines of Nurgle. Following Horus' defeat, Mortarion led his Death Guard in a merry dance of destruction over a score of planets until finally retreating with the remnants of the legion into the Eye of Terror.

Following their retreat from the material universe, it becomes difficult to say with certainty what happened to the Death Guard. However, it seems likely that Mortarion received Nurgle's ultimate reward and became a fully-fledged Daemon Prince, ruling over a Plague world in the Eye of Terror. From this dark and slimy world Mortarion launches fleets of Plague ships into the warp to carry their contagion through the galaxy.