Motto: Death to the False Emperor!
The Black Legion, formerly known as the Luna Wolves and the Sons of Horus, were the XVI Legion of the original twenty Space Marine Legions. Their Primarch was Horus, known to them as Horus Lupercal, and to Imperial history as the instigator of the Horus Heresy, the first great Imperial civil war. Some time after their defeat in the Horus Heresy, they were renamed the 'Black Legion' by their new commander, Ezekyle Abaddon. Previously scattered into countless warbands, they have since unified into a dark state under Abaddon's banner.
The Black Legion was originally known as the XVIth Legion. Instrumental in the Emperor's pacification of the Lunar gene-labs, the legion gained the moniker of the Emperor's Wolves. Upon the discovery of Horus Lupercal, the first Primarch to be rediscovered, the Emperor officially renamed them the Luna Wolves, in honor of their past victories.
Horus was made the Warmaster of the Great Crusade by the Emperor's command, in the aftermath of the Triumph of Ullanor. The Emperor offered Horus the chance to rename the legion to the Sons of Horus, an offer which he accepted at the suggestion of Sanguinius.
Not long after the rechristening, Horus was wounded upon the moon of Davin, and recovered in the Temple of the Serpent Lodge. This warrior lodge turned out to be a Chaos coven, managing to ensnare the Warmaster by Lorgar's scheming.
Horus renounced his oath to the Emperor and turned to the worship of the Chaos Gods, and his sons followed. The part of the Legion that stayed loyal to the Imperium was mostly wiped out on Isstvan III, except for a small contingent that fled on the Death Guard ship Eisenstein.
The corruption of the Sons of Horus spread to eight other legions, as well as parts of the Martian Mechanicum. On Molech, the Warmaster was able to gain much of the same powers as the Emperor.
At the climax of the civil war, Horus struck at Terra itself, and the Imperial Palace was besieged and breached. With news that the Ultramarines were coming, Horus lowered the shields of his flagship in a bid to end the war, daring the Emperor to board it. He did, and the resulting duel killed Horus and crippled the Emperor.
Since the Horus Heresy, the Black Legion has operated within the Eye of Terror under the command of Ezekyle Abaddon. They have launched 13 Black Crusades against the Imperium to rip the Corpse-Emperor from his throne. The current Black Crusade, the 13th, is on hiatus after the fall of Cadia, as Abaddon is plotting an invasion of various other universes.
Since the destruction of Cadia, Abaddon has been restructuring the Black Legion, along with the other Traitor Legions, into a nation of its own, increasingly independent from the wills of the Chaos Gods.
Abaddon has centralized power over the new Black Legionary State around himself. If he dies, it would spell disaster for the Chaos forces, as a succession crisis explodes into civil war.
The Warmaster has put a pause on conquering Holy Terra, and has decided instead to try to invade other universes. His first primary target is The Star Empire of Mechyrdia, as they hold a device known as the Eternity Cascade; Abaddon believes that the components of that device would be of use against the Imperium.
Abaddon is the linchpin holding the State together, and maintains unity between five factions:
The Dark Spiritualists, led by Kor Phaeron, who seek to bring the State closer in line with the wills of the Chaos Gods.
The Abaddonites, loyal to Abaddon and, in case of his death, would likely choose Falkus Kibre as the successor.
The Dark Mechanicum, the ones who control much of the State's industry and technology. They support Perturabo as successor.
The Warmakers, who believe that the Legionary State is stronger when its forces are going out and conquering, pillaging, and burning other worlds. They're too divided to have a successor to support, so they're likely to just ally with another faction, likely the Dark Spiritualists.
The Cursed Ones. Very little is known about them: they worship a minor Chaos entity and seek to destroy all of existence. Even who they're likely to choose as a successor is unknown.