Abaddon
Abaddon (A.K.A: Failbaddon) the Despoiler is Horus' successor and is renowned as the single greatest threat to the Imperium in the galaxy. This says more about the Imperium than Abaddon as his raging incompetence is now well known. He has launched thirteen consecutive Black Crusades against the Imperium, every one of which has failed miserably (by all we mean an overwhelming majority of 13/13). Abaddon somewhat fulfills the role of your typical, Saturday-morning cartoon villain, like Dr. Robotnick from Sonic or Dr. Wily from Megaman: He comes up with some extremely diabolical plan to take over the galaxy/planet, but ends up getting foiled by the hero EVERY fucking time, the only difference is that this is has an obscene amount of grimdark to make his defeat somewhat awesome. Despite this however, the High-lords of Terra still does fear Abaddon as he might unite all of the traitor legions in a single attack that would destroy all of the Imperium......but with his inability to get shit done correctly, Angron is more likely to get this done than Failbaddon.
He also has a model in the tabletop game. It eats everything in front of it in close combat faster than an obese guy would eat his McDonald's supersized meal. Though after you watch this model go down in a single round to a couple of trigger-happy Tau Broadsides, it won't be fun anymore. Fucking railguns. Even more embarrassing is when he somehow manages to get murdered by a Crisis Suit in close combat. Yes, I'm serious. I know what I'm talking about, I'm the guy with the suit that killed him in melee.
His 7th Black Crusade was technically a "success" because he set his goal very very low this time, he basically freaked out Cadia with the threat of a massive invasion but was actually no more than a couple of skirmish runs on certain cities. Ohh yeah and he massacred an unknown number of marines from the Blood Angels, but mostly it was to do with the fact he got an army of Khornate Berzerkers to fuck the Blood Angels over. Success is in quotes because even sending a whole crusade to target a single chapter is sad, especially when the chapter's still around at the end so yeah, it's still a failure by long-term accounts. The closest thing he can come to success is really just like those evil guys in cartoons who just laugh evilly and say "I have won!" even though all they've managed to do is take over Wyoming or something.
His 5th Black Crusade was also technically a success since it managed to wipe out 2 Space Marine chapters, but only with the help of Doombreed, the second angriest daemon entity in the universe. However all he managed to do was burn a city down, which isn't really that much of a feat since there isn't a Chaos Champion who hasn't burned some kind of major populace down either.
He also enjoys calling Horus a weakling and a fool, which is ironic, because Horus actually came close to killing the Emprah and caused the biggest heresy in Imperial history, whereas Abaddon has barely managed to take Cadia(but Cadia is only attacked because Creed is a dick and he could go around it if he wanted) and do anything other than fail, badly.
He has a cool sword though. It's basically a rip off of Archaon's much more awesome and manlier sword; The Slayer of Kings. Failaddon's sword is some strip of chaos blessed metal with an even sissier name called; 'Drach'yen' or some such sissy shit.
Not that he can use it, given his lack of arms.
His full name is Ezekiel Abaddon. Oh, nice GW. Ezekiel, as in the Prophet Ezekyle, Abaddon as in that demon in the Bible. Ha. Ha.
Also, Doombreed > Abaddon. Mostly because he actually succeeded during a Black Crusade and wiped out 2 chapters completely, unlike Abaddon who couldn't even do one. Fitting, considering that Doombreed, being the most ancient Daemon Prince of Khorne, is stronger than all the Daemon Primarchs combine, and perhaps stronger than the Empra himself!! No foul heretic can best our Immortal God-Emperor! Die heretical editor! *BLAM*
The "No Arms" meme
While the exact origin of why /tg/ continues to depict Abaddon without his arms is unclear, this is partly the story. It all started when some drawfag made a picture of Abaddon painting, being the failure that he is he depicted him without arms and so now he fails in the ability to paint.
The other explanation is that Creed stole it during his invasion of Cadia or it was Just as planned.
Another explanation points to a certain dick, who actually fought Abaddon and defeated him (in a White Dwarf issue where Abaddon was trying to wipe out the Ulthwe Seer Council but was driven off, actually defeated/killed alongside his retinue by Eldrad and his council, but 'teleported to safety'), cutting off both of his arms in the process, which further points to Abaddon's incompetency and lack of arms.
But actually, his model's arms are fragile and tend to BREAK ALL THE TIME, FUCK.
Retribution
Abaddon finally gets his big break in Dawn of War II: Retribution. He appears in the Chaos Campaign and serves no purpose than to screech about killing Kyras. The camera is conveniently positioned and several filters are employed in his portrait to avoid showcasing his lack of arms. This would explain why all he ever does is screech. At one point after Eliphas fails to dispatch of Kyras, Abbadon declares how he will make him suffer. Eliphas, no doubt unaware of his armlessness, begs for mercy. Realizing that he can't do anything to him because he has no arms, he quickly let's Eliphas go under the guise that he wants to see him suffer. But we all know better than that.