The Badab War was a major rebellion in the Imperium's history, led by Lugft Huron and the Astral Claws. He first conquered the Badab Sector and decided he should rule all of it, like the Ultramarines ruling Ultramar. Then Imperial forces demanded the tithes. He refused. And three other chapters were honour-bound to fight with him. And thus began the most bloody rebellion in the 41st Millennium. And also the most famous appearances of a shit-ton of minor chapters that most people haven't heard of, who will probably never appear outside a few novel cameos and the Imperial Armour books. Pretty unique among wars in that almost all fights were between Space Marines, with few Guardsmen (traitor or otherwise) in sight.
Many years ago, there was an asshole called Huron, who controlled the Astral Claws and a sector called Badab. With the aid of the Lamenters and Mantis Warriors (Collectively known as the Maelstrom Warders), he ruled the Maelstrom Zone, named because it contained the warp rift known as the Maelstrom. Huron asked the High Lords of Terra to start a crusade into the Maelstrom. They laughed at him, as attacking it would be suicidal. In anger, he withheld his geneseed and planetary tithes, and blockaded the trade routes of other worlds. After 150 years, the High Lords had enough and set a fleet to take the tithe, which Huron fired upon, claiming they threatened him. The local trade lords attempted to send fleets because he was fucking over the economy with his blockades, but they vanished. Due to these events, Huron cut off all ties between the Maelstrom Zone and the outside galaxy, further pissing off the Trade Lords. The Trade Lords then asked the Fire Hawks chapter to sort this mess out. They agreed, seeing an opportunity to settle an old grudge with the Astral Claws.
The Fire Hawks entered the Endymion Cluster, the home of the Mantis Warriors, and were promptly shot down. Thus the Badab War truly began. After some fighting, the Marines Errant and the Executioners came to help the Fire Hawks and Astral Claws respectively. Though the Executioners were loyal, they owed the Astral Claws a favor for saving their asses a few 100 years ago. The Trade Lords and Marines Errant got their asses kicked and ran off. The High Lords noticed this and asked everyone to stop for a moment, but the Astral Claws & Co refused. Thus the High Lords sent the Red Scorpions, Fire Angels, Raptors and Salamanders to destroy the Secessionists.
The Marines Errant and Fire Hawks came back from their asskicking to fight again, aided by the newcomers of the Novamarines. However the Chapter Masters of the Red Scorpions, Mantis Warriors and Astral Claws stopped the fighting for some negotiations. Then for some reason some Chaos dudes jumped out of the huge warp-rift everyone forgot to defend, and exploded the Chapter Masters of the Red Scorpions and Mantis Warriors. Thus the hope for the war ending died. The Howling Griffons, Sons of Medusa and Minotaurs appeared, replacing the Marines Errant who retreated again due to losses.
An Astral Claws Apothecary was captured and interrogated, revealing precious secrets. He was actually a part of the Tiger Claws chapter, who nearly died out and merged with the Astral Claws. The Geneseed tithes were withheld for greater reasons than defiance: Huron was expanding the Astral Claws to beyond Codex Astartes norms. The Exorcists appeared while the Novamarines,Raptors and Howling Griffons were dismissed, officially to combat an Ork invasion, but the truth was that the High Lords thought they were too nice to be trusted to kill off the Secessionists. The Salamanders refused, while the Inquisition kept arguing with them, knowing they were bros with the Executioners and the nicest chapter around. Eventually they gave up trying to talk them into reassigning themselves.
The Minotaurs, using their one and only advantage of numbers boarded the Lamenters' ships. They killed all but 300 Lamenters, who surrendered (really they ran out of bullets, chainswords snapped from over use, and their combat knives dulled to the point of uselessness) like the unlucky bad-asses they are. Nothing really happened for a while, until the Carcharadons/Space Sharks showed up and defeated the Mantis Warriors. The Fire Angels withdrew, due to their heavy losses and hating having to serve with the assholes of the Space Sharks. The Executioners learnt that the Astral Claws were stealing the geneseed of the chapters they killed, and became a rogue element, fighting both loyalist and secessionist. Huron, having fallen deeper into madness over the years, fully seceded from the Imperium, and made the Imperial Cult illegal, as well as all Imperial iconography.
The Star Phantoms appeared, presumably because the author wanted to insert as many chapters as he can into this clusterfuck (or maybe because their planet got sucked into the warp and they were looking for somewhere to crash). The Salamanders convinced the Executioners to stop fighting, and the Siege of Badab Primaris begun. The loyalists tore out a part of a sun and threw it at the planet's orbital defense system known as the Ring of Steel, exploding a hole in it that exposed the space station that controlled it. The Exorcists and Sons of Medusa conquered it, turning the defenses against the Astral Claws in the system. The Exorcists went out to blockade the system, ensuring that no Astral Claws shall escape. The Carcharadons decided to sabotage the planet's reactors, destroying all life on it, while the Star Phantoms, supported by Titans of the Legio Crucius assaulted the Palace of Thorns, the Fortress Monastery of the Astral Claws and Huron's seat of power. Huron killed a captain of the Star Phantoms, but had half of his body destroyed. In the chaos as all the ships retreated from the dying planet, the Exorcists accidentally shot down a few loyalists and let Huron escape. Oops.
In the aftermath, the Star Phantoms became the new rulers of the Badab Sector. The planets and goods of the Mantis Warriors, Lamenters and Executioners were given to the Fire Hawks, Minotaurs and Salamanders respectively, as the secessionists were forced into 100 year penitent crusades. The surviving Astral Claws were executed. The Fire Hawks let the Carcharadons pillage the geneseed and worlds of the Endymion Cluster. The Space Sharks forced the youth to fight to the death and recruited the winners.
Huron was dying, the shattered remains of the Astral Claws that weren't caught and executed retreating into the Maelstrom. He prayed for survival for him and his chapter, and some dudes heard. Years later, a Chaos Space Marine warband known as the Red Corsairs appeared. Their leaders name was Huron Blackheart.
THE END, OR IS IT? (protip: IT'S FAR FROM OVER)
- Astral Claws (Went renegade after they lost and became the Red Corsairs)
- Tiger Claws (Technically, as they were absorbed into the Astral Claws chapter)
- Mantis Warriors
- Lamenters (Figures that they'd join the losing side. And give much better than they got)
- Executioners (Later went rogue and fucked up both sides shit)
- Fire Hawks (Before they became the Legion of the Damned)
- Marines Errant
- Carcharodons (Lead by Tyberos the Red Wake, only known wielder of LIGHTNING CHAINCLAWS)
- Red Scorpions (Forge World's personal favorite chapter)
- Salamanders (The only Chapter in the war that most people would recognise)
- Fire Angels
- Raptors (Mostly notable for their appearance in Warhammer 40,000: Fire Warrior.)
- Minotaurs (Personal favourites of the High Lords of Terra. One of their Captains publicly insulted Marneus Calgar and those who "slavishly fawned" over him. Thus they are awesome.)
- Sons of Medusa
- Howling Griffons
- Star Phantoms
- Titan Legio Crucius (Warmongers)