Nurgle is one of the major Ruinous Powers and is the oldest. The God of Death and Decay has been part of the galaxy since the beginning.
Via our good friends at Lexicanum
His titles include the Plague Father, Fly Lord, Great Corruptor, Plague Lord, Master of Pestilence, Lord of Decay (the translation of his Dark Tongue name, Nurgh-leth) and represents morbidity, disease and physical corruption. Of the four Gods of Chaos, Nurgle is said to be the most involved with the plight of mortals. Those afflicted by his contagions often turn to him in order to escape their suffering. The physical likeness of Nurgle is described as gigantic and bloated with corruption, with foul-coloured, leathery and necrotic skin. Nurgle can also be regarded as the Lord of All, because all things, no matter how solid and permanent they seem, are liable to physical corruption.
All the Chaos Gods are an embodiment of the hopes, fears, and other strong emotions and concepts generated by the mortal races. In Nurgle’s case, the source of power is the living’s fear of inevitable death and disease, and their unconscious response to that fear, which is the “power of life”, the motivating power of mankind and other races. Nurgle and his daemons, in contrast to their putrid appearance, are jovial and friendly in demeanor. His daemon servants and mortal followers usually demonstrate a disturbing joviality and joy at the pestilence that he inflicts, seeing the plagues as gifts and the cries of their victims as gratitude rather than agony. This is demonstrated on the Daemon World of Bubonicus, where an endless chain of crazed revelers circle the planet’s equator in a never-ending dance.
Nurgle is often referred to as Grandfather Nurgle, Father Nurgle or Papa Nurgle by his followers because of his paternal nature. His main enemy is Tzeentch, the Lord of Change, because their power comes from opposing sources. Tzeentch is hope and ambition, while Nurgle is defiance born of despair and hopelessness. Like the other Chaos Gods, Nurgle has a multitude of followers across the galaxy, drawn from all mortal species. The followers of Nurgle often pit themselves against those of Tzeentch in complex political intrigues, forever attempting to mire his schemes for change with dull-minded conservationism and parochial self-interest. Their influence is often successful in thwarting Tzeentch, knowing that whatever survives the collapse into entropy becomes theirs. The church of the Fly Lord is always open to all.Adeptus Mechanicus Is Looking Strong 119