A Shadow over Akeroth
Cult of Damnation
The Cult of Damnation was the original organization that began to aid Nerull in 4A-5A, starting during the Southward March of the Undead Host and ending after the Crimson King was defeated by the Alatian sacrifice.
Aiding Nerull and the Undead Host, the Cult entered areas before the Host infecting the populace with the Dread Plague (helping to create the Dead Scar) and sabotaging the defenses of areas the Host was approaching. When the Cult and the Host crossed the Falmar and entered Garem, the Cult acted as the Magical arm of the Host, helping to combat the Clerics and Wild Mages defending Garem. Ultimately it was the Cult who accomplished the destruction of the Beacon of Light and the ensuement of the Dark Times, with the Host serving primarily as the Muscle (the Cultists acted as commanders of the Undead).
While the original Cult only lasted for about four years, the remains of the Cult would continue to work in the interests of Nerull. Through their actions possibly hundreds of Cults were formed, and countless Demon-Princes were contacted resulting in the dissemination of the Dread Plague further.
No one is sure if the Cult of Damnation is still active, however, many disasters are still blamed on the Cult including everything from Disease to Famine to War. While no one is sure if the Cult continues to exist, it is definitive that the Cult did exist, and that from this Cult many other Cults would come further the cause of Nerull and continuing the cycle of the Apocalypses.