The Long War is a term used by Chaos Space Marines of the Traitor Legions and Renegade Space Marine warbands to refer to their fight to overthrow the Emperor of Mankind and seize control of the Imperium of Man for themselves and for the Chaos Gods. They intend to achieve the final victory that was denied the Warmaster Horus over ten thousand standard years before during the Horus Heresy. The events of the Heresy are nothing more than myth and legends to the people of the Imperium in the late 41st Millennium, but for many of the Chaos Space Marines who dwell within the Eye of Terror where the flow of time moves slower, the terrible passions of that era are still very much alive.
The centuries after the end of the Heresy have seen the Traitor Legions tear themselves apart in the fraction in-fighting that is so common amongst the servants of Chaos. Others have become factional warlords, murder-obsessed berserkers, pleasure-seeking hedonists, Daemonhosts or potent servants of the Dark Gods who are more devoted to achieving ascendance in the Great Game played between the major Chaos Gods in the Realm of Chaos than the struggle against the Corpse Emperor. This is most true of the Primarchs of the Traitor Legions who survived the Heresy, many of whom have become Daemon Princes and given up day-to-day control over their former Legions.
That minority of Chaos Space Marines, including individuals like Abaddon the Despoiler, who have managed to retain their full faculties and kept the corrupting influences of the Warp to a minimum, have done so only by maintaining a sense of purpose dedicated to the final overthrow of the Emperor. In truth, the Long War is the only activity that is capable of reuniting all of the disparate warbands of Chaos Space Marines into a fighting force capable of taking on the forces of the Imperium. It is Abaddon, Horus' successor as the Warmaster of Chaos Undivided, who is most capable of forging this unity, usually so that the Forces of Chaos may unleash yet another one of their Black Crusades out of the Eye of Terror into Imperial space. It should be noted that the Long War is now closer to its end than ever before since the end of the Horus Heresy, due to the limited success won by Abaddon's 13th Black Crusade in 999.M41, which for the first time won control of Imperial space outside of the Eye on the Fortress World of Cadia.
It should be noted that many of those Chaos Space Marines who have managed to maintain themselves relatively free of Chaotic corruption consider that their surviving Primarchs have essentially chosen to forget about the goals of the Long War, preferring instead to pursue the more inscrutable goals of the Great Game. For these Traitor Marines, it is Abaddon, rather than their Daemon Primarchs, who are truly worthy of their loyalty, for in ten thousand years, he has never wavered in his determination to achieve the final victory that eluded Horus.
- Codex: Chaos Space Marines (4th Edition), pg. 17
- Blood Reaver (Novel) by Aaron Dembski-Bowden, pg. 257