Black Legion

"Death to the False Emperor!"

The Black Legion are a legion of Chaos Space Marines from Warhammer 40,000, and are generally the poster boy for the Chaos Space Marines in the series. In the background material for the series, they were once the Luna Wolves, and were the greatest of the Space Marine Legions during the Great Crusade. Their Primarch, Horus Lupercal, who was the first Primarch rediscovered, was eventually made Warmaster of the Great Crusade, and encouraged to rename his legion the "Sons of Horus" to represent his supremacy. However, thanks to some effort on the part of the Word Bearers, Horus was turned to Chaos, taking the Sons with him. After Horus died in the battle of Terra, Ezekyle Abaddon, the First Captain of the Sons of Horus, became Warmaster and renamed them the Black Legion.

They serve as the villain protagonists of the Horus Heresy, particularly the novels Horus Rising, False Gods, and Galaxy in Flames, appearing sporadically after those novels. They also appear in the Grey Knights novel Dark Adeptus, the Night Lords novel Soul Hunter, and the Word Bearers novel Dark Creed. They also feature in the videogame Dawn of War, the table top game Battlefleet Gothic, and the Ultramarines film.

Also check out the character sheet.

Tropes used in Black Legion include:

Big Bad: Horus was this for the Horus Heresy. Abaddon is this for the Black Crusades, and is more or less the top contender for Big Bad of 40K.
City Planet: Their homeworld, Cthonia, was a hive world. Now it's mostly rubble.
Dark Is Evil: As the Black Legion, they're probably the worst of the Chaos Space Marines. Khornate Berserkers? They've got those. Slaanishi Noise Marines? They've got those. Tzeentchian Sorcerers? They've got those. Nurglite Plague Marines? They've got those. And more.
Decapitated Army: Causing this is a Legion specialty, as lightning strikes towards enemy command structures was one of Horus's preferred tactics.
The Dreaded: Abaddon is this to the people of the Imperium.
Elite Mooks: The Justaerin Terminator squad. Although it's unknown it they even kept that name following the Heresy.
Faceless Eye: The Eye of Horus, their symbol, against the eight-pointed star of Chaos Undivided.
Four-Temperament Ensemble: The Mournival (a council of four senior Captains) was designed with this in mind.
- Abaddon is choleric.
- Aximand is melancholic.
- Torgaddon is sanguine.
- Loken is phlegmatic.
From Nobody to Nightmare: Who would have thought a bunch of hive gangers would become the most dangerous force of the 41st millennium?
General Failure: Despite being the heir of Horus, in ten thousand years Abaddon has failed to conquer the Imperium. Somewhere along the way, however, he wised up and started focusing on The Long Game, acquiring superweapons of vast power. His failures have not gone unnoticed by the other Chaos Legions, and are addressed in both Soul Hunter and Dark Creed (where Erebus of the Word Bearers goes so far as to mention that there is talk of needing a new Warmaster).
Generation Xerox: One unique quirk of the Luna Wolves gene-seed is that they have a higher percentage of physically resembling Horus. Aximand was the most notable example, and was nicknamed "Little Horus" because of it. These individuals were referred to as the "sons of Horus", which is where the idea for the name came from.
The Horde: Leadership was always flexible within the XVI Legion, and warbands roam the galaxy following the will of their Chaos Lord. They still band together whenever Abaddon orders a Black Crusade, however.
Jack of All Stats: They're the most balanced of the Chaos Space Marine Legions.
Light Is Good: When they were the Luna Wolves, they were the best of the Emperor's legions. No one could stop them.
Like a Badass Out of Hell: The Black Crusades.
Lost Technology: It's mentioned in Dark Creed that Abaddon keeps members of the Dark Mechanicus in line by his massive hordes of archaeotech.
The Magnificent: The Black Legion seems to thrive on this: Abaddon the Despoiler, Malogurst the Twisted, Ygethmor the Deceiver, Araghast the Pillager.
Meaningful Rename: From Luna Wolves to the Sons of Horus during the Horus Heresy, to represent Horus's rise to preeminence, to the Black Legion following Horus's death. And when Horus goes traitor, those members of the XVI Legion who remained loyal to the Emperor renamed themselves the Luna Wolves.
The Mothership: The Vengeful Spirit, the Legion's flagship from the Great Crusade until the Gothic War when Abaddon made the Planet Killer his flagship. The Vengeful Spirit is still his personal vessel, however, as shown in Soul Hunter.
Number Two: Abaddon and Maloghurst split the role between them for Horus during the Horus Heresy, with Abaddon being The Dragon and Maloghurst being The Consigliere.
The Purge: The virus bombing of Istvaan III in Galaxy in Flames, where the Sons of Horus, the Emperor's Children, the Death Guard, and the World Eaters all kill the Astartes who were more likely to remain loyal to the Emperor over their Primarchs.
The Remnant: Though they're the largest of the Traitor Legions, in both Soul Hunter and Dark Creed it's stated that they're a far cry from what they once were.
We Have Reserves: Unlike other Traitor Legions, the Black Legion actually has growing numbers, as many Space Marines defect to the Black Legion for power and advancement. In Dark Creed it's mentioned that the Black Legion outnumber the Word Bearers nearly ten to one.
Would Not Shoot a Civilian: When they were the Luna Wolves, one of their specialties was minimizing civilian casualties. Following their fall to Chaos, they frequently and gleefully subvert this trope.