Bloodthirsters are the greater daemons of Khorne. They're fuckhueg daemons with wings who carry a giant axe (Or more rarely, a flaming sword) into battle that's capable of hacking platoons of soldiers into mincemeat at a moment's notice. They are also filled with rage and generally considered the most dangerous entity in Warhammer 40,000 that isn't a god or comparable to an Imperial war titan. Yes, you read that right. In all the grimdark violence of 40k, a Bloodthirster is the single most dangerous being to walk the galaxy, baring singular named figures such as primarchs, C'tan, etc. That is how fucking dangerous these things are. In Warhammer Fantasy they are somewhat less impressive, as the scale of the game puts Chaos on a much lower pedestal than in 40k. Regardless, a Bloodthirster is still nothing to take lightly.
Just to be clear, however, on HOW FUCKING DEADLY bloodthirsters are, Ka'Bandha is a third rank Bloodthirster, which means that he is at least two orders of magnitude beneath the greatest of Khorne's Bloodthirsters, and only the two most powerful incarnates were able to put up a good fight. And in 40k, number crunching there are 19,173,960 bloodthirsters hanging about in the warp, and one was capable of taking on (or at least not being immediately swatted aside) a Primarch (and Sanguinius, at that). All of them entering the universe at once is game over. On the other hand An'ggrath is supposed to be a first rank Bloodthirster theoretically on par with fucking Angron and yet he lost a duel to a mortal with a glowy stick.
To be fair, a Bloodthirster is the Blood God's avatar of his rage, it will maim, kill, burn, split open, break the back of their opponent, drink the blood from their hearts, mutilate, anything that would sate their never-ending battle lust and to glorify Khorne. They are capable of killing virtually anything with their massive axe and strength and they are nearly impossible to stop without at least bringing something the size of a Baneblade with similar firepower or the Daemonhunters.
In The End Times, Bloodthirsters underwent a similar treatment to Verminlords in that they got split into several varieties; the Bloodthirster of Unfettered Fury (eighth host, the weakest of them all... if you can call WS10, S6, I9 and A6 [+1 for wielding two hand weapons) weak), the Bloodthirster of Insensate Rage (6th legion, monster-killers, so you're paying +100pts over the Unfettered Fury for a greatweapon and Heroic Killing Blow) and the Wrath of Khorne (third legion, 550 points by is Stubborn, has Magic Resistance 3, has a S5 Flaming breath weapon, Relentless Hunter [Hatred: Characters, adds +3" to charge if charging a character or unit with a character), and wields the Wrathaxe & Bloodflail combo (+2 Strength in first round, +1 attack in all rounds).
Bloodthirsters seem to have inherited the mantle of "that thing that gets beaten up to show how cool a dude is" from the Avatar of Khaine, though that current has always been present to some degree. As their own lore pumps them up to such high degrees, fluff writers inevitably seek to tear them down to show off how badass their characters are. Leading to many Bloodthirsters getting defeated in often humiliating fashions, like Skarbrand getting cut in half with a single swing from Dante, an Ultramarines Chaplain beating one to the ground with ease in winter assault while dozens of other Bloodthirsters are shot and beaten back into the warp throughout Dawn of War and its expansions, Nagash crushing the throat of another Bloodthirster and choking it to death, Skarbrand getting literally one-shotted by Malekith before Malekith started collecting power ups like candy, Ka'Bandha embarrassing himself against the Sanguinor and getting Bane'd to death by Sanguinius and more.
To be fair, like all daemons, the power of Bloodthirsters is not consistent and restricted based on what is powering them in the physical world. In addition, unlike the Avatars of Khaine there is a lot more fluff of Bloodthirsters slaughtering armies and killing the populations of entire nations, so they get to show their credentials. Also, some of those defeats were justified (ie Nagash was an Incarnate and had a giant-sized new body when he fought against Ka'bandha and Malekith had magic and his gigantic dragon steed helping him the fight against Skarbrand). As for 40k, well, the Imperial Guard has a lot of artillery and tanks.
The Sisters of Silence, able to see daemons for what they truly are, have started a charity drive to educate the Inquisition and similar organizations on the soft, helpless side of Bloodthirsters. They hope to contact all Grey Knights and Daemonhunters before the end of the 42nd millennium, in order to prevent the senseless destruction of these endangered creatures by every protagonist in every novel ever written. Every nine seconds a Bloosthirster is punched in half by a Guardsman, and this isn't "someone else's problem". If you don't act now, how long will it be until a Bloodthirster you know (or love) is slain?
- An'ggrath: Khorne's most favored Bloodthirster, created to replace Skarbrand. A Greater Daemon for Apocalypse made to rape anything that he gets into melee with. But his fluff sucks as three of the four mentions of him in fluff are of him losing to: Lorgar, the weakest Primarch (which, it bears mentioning, is still more than enough to slap the shit of high-ranking greater daemons); an Inquisitor Lord, albeit a badass one (who called him a slave and told him to beg Khorne for mercy when he next saw him), and Calgar, who crushed his head when he clapped his hand, which isn't discussed as much.
- Daemon of Tartarus: A Bloodthirster (or maybe a Daemon Prince, who knows) sealed in the Maledictum artifact by the Eldar of Craftworld Ulthwé. SSSIIINNNDDDRRRIII Myr attempted to use it to achieve Daemon Princehood, but was thwarted by Gabriel Angelos, who destroyed the Maledictum and ended up releasing it. Later spent several years working with Azariah Kyras in trying to destroy Subsector Aurelia in a mass sacrifice to Khorne. Began merging with Kyras on Cyrene, where it was bombarded from orbit by the Blood Ravens. Or the Imperial Guard. Or the Eldar of Ulthwe. Look, we're still not sure who did it, but we know it's dead. Or at least banished for some centuries (the daemon, not Kyras). Actually, not much is known about him period, he only has one speaking role, during which for a daemon of Khorne, he seems rather calm and polite. For a Daemon of Khorne, he's also very indirect and extremely manipulative, only once actually engaging in direct battle in the final battle at Cyrene; and even then he was only half of the equation; merged with Kyras as he was. Otherwise, all the blood he spills for Khorne on screen was through the result of manipulating others into spilling blood pointlessly, from the war for Tartarus to the Aurelian Crusades. So he's the most dangerous possible kind of Khornate, the intelligent kind, one who doesn't give you a chance to stop his bloodshed because you don't know he's behind it until it's almost too late. And of course, a Khornate who is actually capable of corrupting others, whereas most can at worst kill you and swallow your soul and yaddah yaddah yaddah. Notably referred to as Khorne's Herald and Messenger, which in feudal hierarchies has always been a high ranking and very posh job with a lot of benefits to it, likely making him a first order Bloodthirster or one of Khorne's favourite Daemon Princes. Also has a rather unique appearance, his head being a flaming horned skull rather than a cow head, an angry horned person, or a screaming red maw. But of course, what's more Khornate than a burning skull? If someone could draw a Bloodthirster that's a flaming skeleton, that'd be awesome.
- Ka'Bandha: The greatest of Khorne's servants. he who broke Sanguinius's legs at Signus Prime. Sanguinius later paid him back during the Siege of Terra (if you don't count the time he got payback on Signus Prime) by breaking Ka'Bandha's spine over his knee. He may or may not be the cause of the Black Rage. Reappeared to troll the Blood Angels in M41. Actually, every time he goes up against them he embarrasses himself. The Sanguinor once bodyslammed him from orbit. In Warhammer Fantasy, he is the greatest of Khorne's huntsmen and comes across as possibly the strongest being in the setting who isn't a god. So powerful in fact, that he took on up to SIX Incarnates at once and still kept going. He even kills Caradryan, incarnate of fire, has the wind of fire unleashed into his flesh, and still keeps going. The only incarnates who matched him in strength were Nagash, who fought him to a standstill, and Sigmar, who is a literal god so of course he ended up winning that fight.
- Skarbrand: Once the greatest servant of Khorne, until Tzeentch tricked him into even greater acts of wanton destruction to the point he challenged Khorne himself as he was so angry. Khorne was so pissed that he throttled the emotions out of him, except for rage and threw him so hard that he flew across the Warp for a straight week thus banishing him for pissing Khorne off. Tzeentch like always, just sat on his throne, conjured up a martini glass filled with his poison of choice, and muttered: Just As Planned. Unfortunately, he now kills more in one day than he ever did when Khorne still liked him, so this could be one of the few times Tzeentch played into Khorne's hands. Just As Planned. He is thought to be Khorne's truest servant, since he kills without purpose and can gain no favor. Though he continues to fight for Khorne, it is for nothing since Khorne has no mercy in his heart, and his exile is eternal: None more closely embody the ideal of senseless slaughter for carnage's own sake. He now has a very pretty, very expensive model; $200 AUD [$130 USD] for one guy!? Nagash does more for the game in crunch and lore and he costs less. Makes you feel sorry for Aussie tabletop fans. Apparently being in exiled from Khorne-land gives Skary a lot of time to pretty himself up. In Age of Skubmar, Skarbrand continues doing his thing; killing for the sake of killing. Eventually, however, Archaon gets so pissed off with Skarbrand killing his dudes when Khorne unleashes Skarbrand that he basically scolds Khorne with "I can't win the Realms for Chaos if Skarbrand keeps soloing entire armies", Khorne, despite not enjoying being told what to do, saw this as the perfect way to punish Skarbrand even MORE. What's worse than not being able to feel anything other than hate and rage? Not being able to express it. So Khorne makes a thing called Brass Chain to tie up Skarbrand in some mega-fortress in the Realms and for added lulz, gives Archaon the power to unleash/lock up (though does require Gaunt Summoners to do so) Skarbrand. Despite being locked up, Skarbrand regularly kills entire armies. In Balance of Power, he's thrown by Khorne to capture Alarielle: The Emo Queen, only to be thwarted by Grey Knights, I mean, Lizardmen, I mean Seraphon. Who proceed to kill everything else that came with Skarbrand, until just Skarbrand is left, who then 'kills' the entire Slann's army, at which point the Slann decides not to bother trying to kill Skarbrand and returns him to his jail cell... Without locking him back up. Right when Sigmarines were trying to break in to smash the chain forever with Fire Dorfs to prevent Skarbrand ever being controlled again. They failed to do so, but at least had the consolation prize of Skarbrand destroying the entire garrison before somebody tells Archaon he's loose. Must be suffering being a bitch Bloodthirster.
- Kazarkos: A sixth-ranked Bloodthirster from Age of Sigmar with an extreme fixation on butchering the Seraphon (Lizardmen) and Disciples of Tzeentch. The latter of which stems from his heated rivalry with the Lord of Change F'tanax, though how this rivalry began was never stated. Kazarkos led a centuries long massacre in the ashen steppes of Arida in the Realm of Fire before being felled by a Saurus Scar-Veteran. Such a defeat led to his banishment from the Mortal Realms and gave him a seething need for revenge against the lizard-folk. It was the dark master Be'lakor that would release him, binding the Bloodthirster to his will and using him to destroy the prophetic Old One device called Tepok’s Eye. As fate would have it, he had to work with the also enslaved to Be'lakor greater daemon F'tanax, and the two were quick to fall onto each other in a gory display of daemonic savagery. Though the first prince actually wanted this to happen, as it would make the whole attack look like a mindless feud between fractious chaos hordes instead of his coordinated scheme. Kazarkos is still in service to Be'lakor, and he’s not happy about it, though as long as he can claim Saurus, Skink, and Slann skulls for Khorne, he is complacent.
There is confusion caused by conflicts in the canon over whether An'ggrath or Ka'Bandha is the most powerful/greatest/simply best of Khorne's Bloodthirsters, as there is proof to support either side. Don't even get the canon-keepers started on Skarbrand when he had wings. Considering how nutty Chaos actually is, such a confused state of being is hardly unexpected. It could also be GW and Black Library writers just don't talk to each other and get some common facts straight. This could be actually fixed if the two battled, but Khorne himself will never allow for this no matter how tempting that would be. Thus we'll never know. In Warhammer Fantasy, it was pretty clear who was the strongest. An'ggrath never gets a mention, while Skarbrand is defeated pretty quickly by Malekith (note this happened before he became either the Phoenix King or the Incarnate of Shadow) though Malekith had some help from his dragon. Ka'Bandha, on the other hand, proves to be a nearly unstoppable rapetrain that takes on nearly all the Incarnates (other than Grimgor) over the span of one battle, smashes anyone else and only even expresses pain when the ENTIRE WIND OF AQSHY scorches him with the death of Caradryan. He is only stalemated by Nagash (admittedly after Tyrion broke Ka'Bandha's axe), and finally defeated by Sigmar Ascendant.