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human
[Wh40k] Why does anyone still follow Abaddon the Despoiler despite his continuing track record of failure? Why do the Chaos Gods continue to grant him unanimous favour?
gpt
Who said they were failures? Not all the Black Crusades were about OVERTHROW THE IMPERIUM.

1st Black Crusade: Abaddon recovered the Daemonsword Drach'nyen. Abaddon's power swelled to inhuman proportions and he was recognised as Horus Lupercal's true successor amongst the Traitor Legions, the new Warmaster of Chaos Undivided.  Mission Accomplished.

2nd Black Crusade: Abaddon places a terrible curse upon the worlds of the Belis Corona Sector, infusing them with the touch of the Warp. Meanwhile, his fleet attacks the sector's Imperial shipyards, destroying dozens of Imperial Cruisers being constructed or repaired. Before the Imperium can muster its strength to fight back, the Despoiler retreats into the Eye of Terror, his works complete. Mission Accomplished.

3rd Black Crusade (909.M32) - The Despoiler sends the Daemon Prince Tallomin against the Cadian Gate in a reckless and bloody frontal assault. Under the cover of the attack, Abaddon leads a dedicated strike force to desecrate the Shrine World of Gerstahl, breaking the ancient seal on the Imperial saint's tomb and utterly destroying his remains whilst the Imperium's attention is elsewhere. Mission Accomplished.

4th Black Crusade: Also known as the El'Phanor War, during this Black Crusade the Black Legion besieges the great Citadel of Kromarch. Abaddon leads the charge against the adamantium gates of the famously unbreachable citadel upon the world of El'Phanor. In an orgy of violence, the Traitor Legions and their daemonic allies fall upon Kromarch and his kin, extinguishing their ancient line forever. Mission Accomplished.

5th Black Crusade: Abaddon scours the Elysia Sector, raiding hundreds of worlds and causing untold death and destruction. On Tarinth, the Despoiler lures the Warhawks and Venerators Chapters into the ruins of Kasyr Lutein where he traps them between the daemonic hordes of Khorne and the forces of the Black Legion. In a bitter last stand, both Loyalist Chapters are utterly destroyed, their skulls taken by the ancient Daemon Prince Doombreed, whose existence precedes that of the Imperium itself, to be mounted upon the mighty throne of Khorne. Mission Accomplished.

6th Black Crusade: The Sons of the Eye were a splinter warband of the Black Legion. Always jealous of the Primarch Horus' favour for the Despoiler, the Sons of the Eye considered Abaddon a usurper. During the 6th Black Crusade launched in 901.M36, Abaddon visited his wrath upon Drecarth the Sightless and his treacherous warband. Abaddon aided the Sons of the Eye in an assault on the Forge World of Arkreach, fighting side-by-side with them until the Adeptus Mechanicus defenders were finally crushed. The triumphant Abaddon then slowly killed Drecarth, impaling him with the Talon of Horus, even as the Despoiler made him watch every member of the Sons of the Eye bow before their new master. Mission Accomplished.

7th Black Crusade: This one is technically a failure, while the forces of chaos were unable to break the Imperium's defenses in a series of guerilla wars, Abaddon was able to almost annihilate the Blood Angels chapter. Draw/failure.

8th Black Crusade: Abaddon the Despoiler completes a complex ritual of death in the name of Tzeentch, the Changer of Ways. On worlds throughout the Segmentum Obscurus, Imperial citizens are slaughtered in precise numbers and esoteric rituals. Only when the Inquisition finally breaks the code of damnation is the Imperium able to bring an end to the Black Legion's rampage across the Segmentum, but not before countless worlds have been saturated in death. Mission Accomplished.

9th Black Crusade: The 9th Black Crusade was a campaign led by the infamous Chaos Lord Abaddon the Despoiler and his Black Legion, launched out of the Eye of Terror in 537.M38. Intending to bring about the demise of the naval fortress of Cancephalus, Abaddon lead his forces against the heaving populations of the nearby world of Antecanis. Monarchhive, the hive city that served as the seat of Antecanis' Imperial Commander, was besieged by Abaddon's vanguard. The Black Legionaries burned and butchered their way through the lower levels, whilst Abaddon himself stormed its inner sanctums. The hive populations were slowly destroyed by the slaughter. By the time the forces of the Imperial Guard arrived from Cancephalus to reinforce the desperate survivors, Abaddon and his favoured Legionaries had already left orbit, dropping a dozen Cyclonic Warheads onto the ruins of Monarchhive in a last gesture of contempt. The seventeen-year-long war that ensued robbed both Antecanis and Cancephalus of their most precious resource -- manpower. Without the fleets of Cancephalus to stop him, Abaddon is able to ravage the rest of the sector at will. Mission Accomplished.

10th Black Crusade: A joint campaign carried out by Abaddon and the Daemon Primarch Perturabo. Both the Black Legion and the Iron Warriors struck against the worlds of the Scarus Sector's Helican Sub-sector. While the Black Legion attacked the capital world of the sub-sector, the Hive World of Thracian Primaris, the Iron Warriors focused their own assault against the Iron Hands Chapter to fulfil an old blood debt. In a series of bitter sieges, the Iron Warriors burned the Medusa System's outlying worlds and pushed the Iron Hands to the brink of destruction. Only when the Loyalist Chapter receives heavy Imperial reinforcements did Abaddon and the Iron Warriors retreat back to the Eye of Terror, taking with them valuable information about Medusa's defences for use at a later date. Draw.

11th Black Crusade: Abaddon the Despoiler launched his 11th Black Crusade out of the Eye of Terror. In an attempt to break through the cordon of Warp Storms surrounding the Eye into Imperial space, he employed a captured daemon to navigate. The effort proved fruitless, and instead the daemon warped the Black Fleet directly into the path of the Orks of WAAAGH! Murgor. In the ruins of the Cardinal World of Relorria, the Black Legion brought the Orks to battle, Bolter rounds and Chainswords tearing into the green-skinned xenos. After months of bloody warfare, Abaddon decided to leave Relorria to its fate and the Black Legion returned to the nightmare realm of the Eye of Terror -- but not before the Warmaster of Chaos filled the holds of his Black Fleet with a multitude of captured Orkoid specimens for later experimentation. 

12th Black Crusade (139.M41) - Better known as the Gothic War, this Black Crusade was fought for control of the Gothic Sector in the Segmentum Obscurus between the Chaotic forces of Abaddon the Despoiler and the defenders of the Imperium. Abaddon lead his great fleet into the Gothic Sector, capturing two of the prehistoric xenos-built star forts known as the Blackstone Fortresses. Under Abaddon's control, the might of the Blackstone Fortresses is used to destroy entire worlds in the Gothic Sector during this campaign. The Despoiler's fleet is driven back at great cost, though Abaddon escapes with the Blackstone Fortresses into the Eye of Terror. The vast engines of destruction emerge once more into the material universe during Abaddon's 13th Black Crusade over eight hundred standard years later. Mission Accomplished.

13th Black Crusade: Well, he blew up Cadia, so he's not doing bad so far.

As you can see, only a couple Black Crusades could really be considered "Failures". They each had a specific objective that Abaddon was trying to achieve and for the most part he was successful. Most of the earlier crusades were carried out for Abaddon to consolidate his power within the Eye, while in the later crusades he was gathering the tools he needed to bring the Imperium down. It is only the latest crusade that has the ultimate objective of bringing down the Imperium forever.