"Had it been 11000 thousand years earlier, and this sucker came for a visit, you would be voiding your bowels, witnessing a literal satan strolling through your church. Yes, and this nice young man would like you to distribute mandatory alms to the Church of MegaSatan by ripping out your jugular and pouring out its contents in a mile wide gore-lake."
Bloodletters are Khorne's foot soldiers. While getting to a battle they march as a unit, but when they get into the battle they go fuck-crazy with rage and actually see it as a competition amongst themselves to fuck up the enemy worse than the other Bloodletters. They have rage-faces and bodies that make them look like Halo 3 Elites (just look at their legs). Like most Khornate Daemons, they have a sense of pride and will usually fight bravely, ferociously, and honorably, but they sometimes resort to cheap-ass Tzeentch-esque tactics (i.e. cunt-punching a Bolter Bitch).
Bloodletters wield a Demonic blade known as a Hellblade which is given to them by Khorne himself and is so sharp that wounds inflicted by a chainsword look like paper cuts by comparison. Khorne's favorites get to ride Juggernauts, these special chosen called Bloodcrushers. Especially awesome Bloodletters are allowed to crew Skull Cannons, or strangely enough be the team to a Blood Throne chariot.
The most famous Bloodletter is imaginatively named Skulltaker, and is so bad-ass that Khorne occasionally allows him to keep the skulls of his most challenging opponents as trophies rather than adding them to the Skull Throne; seeing as these victims include several Grey Knight captains (however the hell that happened), an Ork Warboss, and seventeen Eldar Exarchs, this seems only fair.
Despite being made of awesome sauce, they are for unknown reasons not as popular with fa/tg/uys as their Slaaneshi equivalent, hence why that page has over 9000 images and this page has few. Despite the fact most players will field more Plaguebearers however, Bloodletters are still more popular.
An ironic kinship they share with Daemonettes is how their models evolved. They started out as basically scaly, tailless Xenomorphs (you know, those guys from those flicks with Sigourney Weaver as Ellen Ripley?) with horns, underwent a change into more "conventional" big red spiky devils during the edition in which Daemonettes got remodelled into multi-boobed mantis-armed succubi, then went back to an updated version of their old look the next edition. Also, penis-headed Bloodletters carry huge swords, but spiky devil Bloodletters carry giant axes - what's up with that?
Herald of Khorne
Basically what happens when a lesser daemon earns an upgrade by basically being the teacher's pet. Daemonic Heralds are exalted Lesser Daemons who have gained the favor of their Gods, often leading Daemonic armies into battle.
In the case for Khorne's pet psychos. Heralds of Khorne lead the legions into battle. They carry on the traditions of Warhammer Fantasy and make a mockery of the Imperial Guard by marching in disciplined regiments accompanied by horns (the marching kind, not the spikey kind) and war drums (oh so Ork-y); of course, that Imperial-ish manner goes to the warp when they charge into battle, going into a battle-rage and getting shit done.
As such, they are the largest and most senior Bloodletters. They are rampaging masters of combat, capable of massacring squads of lower fighters. It is said that upon selection they are set against other aspiring Heralds in a vast arena of the Brass Citadel in the Realm of Chaos known as the Skullpit, forced to participate in a contest of champions to wean out the undeserving. The energies of those who fall are reclaimed by Khorne and given to those that remain. Thus the victor becomes swollen with power, and their Hellblade, having leeched a measure of this new strength, becomes known henceforth as a Blade of Blood.
Heralds of Khorne often tame and ride Juggernauts into battle or mount a Blood Throne of Khorne. The Greatest Heralds of Khorne are known as Sacred Executioners. Other variants include Rendmasters, Skullmasters, and Bloodmasters.
Rendmasters are a type of Herald of Khorne, who ride atop a Blood Throne and as such, are absolute murdermachines with a kill count that can rival good Kharn. Unsurprisingly, such acts are needed to be committed to earn that honor.
High in the Blood God's favor, a Rendmaster embodies a locus of Khorne's power and his blessing ripples outward from the Blood Throne. So does a portion of Khorne's unbridled wrath become infused within the veins of nearby Daemons, lending ferocity to their blows and driving them into a maddened frenzy. Thus, a conflict fought in the Blood Throne's shadow is inevitably an example of battle at its hardest and most unforgiving.
Despite their high standing however, Rendmasters constantly prowl battlefields like lions on the prowl, searching for those foes whose plundered skulls would make the most audacious offerings to Khorne.
Skullmasters are a type of Herald of Khorne, who favor the headlong charge of daemonic cavalry and ride atop Juggernauts. They are in affect, a Bloodcrusher leader. Like the Rendmaster, the Skullmasters are an elite unit of merciless slaughter.
They are so called Skullmasters as they have a overt hardon for....well....skulls. Seriously, these guys get a immodest erection when they relish the sensation of skulls shattering beneath brazen hooves, and will thunder into the thick of battle to seek out the worthiest opponents. How lewd Khorne, must have taken some lessons from Slaanesh. You can't fool us Big K, we know you're tsundere for him/her/it.
Skullmasters are most often found in the Blood God's armies leading a cohort of a Brazen Thunder Legion, and in battle they are always at the forefront of a Bloodcrusher cavalry charge. What foes their Daemonic mounts do not gore or crush underfoot are quickly dispatched by the Skullmaster's Blade of Blood.
Bloodmasters are a type of Herald of Khorne, who are rampaging masters of combat and earned their titles by performing particularly noteworthy acts of violence. Unlike the other Khornate Heralds, these guys ditch the mounted platforms and go in deep and dirty.
In battle they delight in decapitating their prey and plunging their wailing Blade of Blood deep into the beating hearts of their enemies and then ripping it out, Mortal Kombat style. However a Bloodmaster cannot simply give in to its desire for slaughter, or else they would glory hog all of the skulls and what honorable Khorne follower would like that type of guy?
Their primary role on the battlefield is as a director of the massacre against the Blood God's foes. To this end, these Heralds imbue the Daemons of the cohort they lead with a portion of their own eternal malice, heightening the inherent bloodlust of the minions of Khorne to near autistic levels.
How Khornettes are depicted on /tg/.
Notably, Orks canonically get the same hormonal incentives for battle that humans do for sex (presumably on top of the hormonal incentives that humans get for battle), so an Ork would probably look at a Bloodletter the same way /tg/ does a daemonette.