An'ggrath the Unbound, the Guardian of the Throne of Skulls, Most Favoured of Khorne, Lord of Bloodthirsters and the Deathbringer, is the most powerful of Khorne's greater daemons, and should by rights be a hard-ass bastard who would reduce neckbeards to manly tears and gnashing teeth of jealously at his bad-assness.
Instead, in an hilarious example of fail, he suffers from a severe case of Avatar of Khaine syndrome: almost every time he has been mentioned in fluff, he has been soundly beaten and sent running back to his master with his tail between his legs. Perhaps he represents the concept of loss in war? No idea, but it would go a long way to explain why he sucks at winning.
During the Horus Heresy, Lorgar, weakest of the Primarchs (maybe), had to face An'ggrath, in a duel to prove his loyalty to the Chaos Gods. You'd expect Lorgar, who got kicked about by his brothers, to be well and truly curb stomped here but no, he gave An'ggrath a well resounding butt kicking instead. Did my cup of tea just fall from my hand in surprise? I think it bloody well did. Clearly the plot armour here is strong.
The next time he was summoned into the material plane was on the hive world of Ashur where Khornate cults were being purged by the Metamarines chapter. In desperation they invoked An'ggrath's name and summoned him, where he proceeded to BUTCHER EVERY LIVING THING ON THE PLANET. It took him 7 years to do so, which some would think is too long for such an angry winged tower of strength and savagery with an axe and whip, but consider that that he has to chop his way through entire hive cities' worth of Guardsmen and gangers, as well as an entire chapter of Space Marines (its actually pretty fast, as it would take 50 kills per second on average to deal with 10 billion people in 7 years) Add in that he has to fly from hive to hive, and it could very well take him those 7 years, after which he just disappears. Therefore he was not defeated and banished back into the warp, but simply ran out of things to chop apart.
The third time he was summoned was during the Siege of Vraks. Towards the end of the conflict, An'ggrath finally decided to show his face, perhaps hoping to steal some of the glory while no-one was watching. Unfortunately for him, the Inquisition got a whiff he was creeping around and one of their Inquisitors, armed with a potent toothpick, promptly pricked An'ggrath and sent him yelping back to Khorne's throne (although to be fair, the toothpick was made specifically to kill ultra-daemons, so that helped Inquisitor Hector Rex cut off An'ggraths knackers).
The last time he was summoned, he fought Marneus Calgar in combat. Given how the previous people who beat him were a fucking Primarch and a highly augmented psyker Inquisitor - who was armed with a sword specifically meant to kill super-daemons like him, who had a shield meant for dealing with attacks like his, who had actually had an audience with the Emperor (enough so that it apparently gave him some sort of aura) and who knew An'ggrath's true name - you'd think a Space Marine who had nothing the other two did would fare no better than the Metamarines or the Grey Knights he'd ripped apart like they were nothing. Of course, this being Marneus Calgar, he just clapped his hands together on An'ggrath's head and the daemon's head exploded, immediately killing the super-fuckup. Fucking serious, turns out it was that fucking easy all this time.
The Blood God did sigh and put his head in his hands, shaking it with despair. What is an evil god to do when the best he had is something that cannot even take out one,
unaugmented HIGHLY AUGMENTED, unsupported Inquisitor?
Well, call Forge World of course! Thus they made a model (though good ol' GW has made a cooler one) and rules for him. To be honest, he just looks like a normal Bloodthirster pumped full of steroids. Don't do drugs kids! You'll end up all red, veiny and unable to do anything but RAAAGGGEEEEEEE.
Canon is shifty on whether Ka'bandha or An'ggrath is in fact the most powerful Bloodthirster, as multiple conflicting sources and titles each state that one or the other is in fact more full of RIP AND TEAR. Matt Ward favoured the former, a clear sign you should favour the latter (win/loss ratio notwithstanding).
On the Tabletop
At a point value of 888, this model is incredibly difficult to kill, with T8, 24 wounds and a 2+/4++, not to mention that he can Deny two psychic powers and gets +3 to any Deny rolls he makes. He's a pure beatstick, with his only aura simply allowing Khorne units within 9" to use his leadership of 10. However, he is one hell of a murder machine, striking at S16 AP-4 on the charge with d6 damage and 11 attacks, though his attacks, strength, and movement go down as he takes damage (although it's pretty hard to damage him in the first place). Any units he charges into combat with take d3 mortal wounds on a 2+. On a 4+, any enemy units within d6" when he dies (IF he dies) take d6 mortal wounds. As icing on the cake, he has an assault 2d6 Bloodlash (plasma pistol profile with 2 damage) which can be fired into and out of combat. Literally nothing in the game can stand up to him for long.
With the introduction of the Daemons codex, a detachment with him alone means he now can re-roll his charge distance and take a FNP warlord trait, maxing out his survivability and damage potential. This is further exacerbated by liberal uses of the Warp Surge stratagem to give him a 3++, Frenetic Bloodlust to fight again, and Locus of Wrath if you absolutely have to kill whatever you are fighting. Your only limit is the amount of CP you can scrounge together with the rest of your army.
Like all the uber-daemons, he can be summoned with an "arch-daemonic ritual", where the summoner rolls nine dice to try to reach his power level. Doubles inflict mortal wounds on the summoning character, while triples kill them outright (but the ritual can still succeed).
Interestingly enough, back when he was first released An'ggrath can and would get his shit kicked in by Hector Rex, just like in the lore. Unlike in the lore however, the reason was because Hector could allocate all of the wounds dealt to him to his acolytes, which he could have 10 of - 9 of which could be carrying Storm Shields. Hector's gear knocking An'ggrath's stats down, him using an anointed force weapon (always wounding An'ggrath on a 4+), as well as being able to re-roll all misses and failed rolls to wound don't help big red either. Thanks to this, it takes An'ggrath 10 rounds of combat to actually kill Rex, whereas it only takes Rex 5 rounds to kill An'ggrath.