"Does our most holy father reward those who take the easy road? Does he traffic in temptation? Of course not! Sigmar rewards hard work, resolve in the face of temptation, and, above all, courage when confronting mortal danger. My friends, the easy path leads to corruption... to damnation... to perdition. Only through suffering can you ever see the glory that is the truth, that is the founder of our glorious Empire, that is our divine father; Sigmar!"
- – Rolf von Steulden, Flagellant. Also, manliness at its finest. Ministorum Priest pray the emperor to sound so fantastic in their speeches.
Flagellants are smelly religious hobos who have different origins and ideals but one thing in common...purge every fucking heretic who dares to stand within a 400 miles radius from them. They roam around the Empire, spreading their belief, gathering followers and helping out the empire army with the occasional brawl against goatfuckers, vikings raiders and undead hordes.
Old lore also had a Chaos equivalant in the diseased flagellants, basically people who caught some manner of incurable disease and were exiled from their homes so they started wandering around, whipping themselves in hopes of getting Nurgle's mercy.
Real World inspiration
During the Black Death a number of people in Europe got it into their heads that the fact that everyone dying of plague was a sign of god's wrath against man's sinfulness and decided that self punishment would convince the almighty to lay it off with the massive death and suffering of the innocent. They'd wander from town to town in a conga line chanting and whipping each other with cats of nine tails, often picking up fellows as they went. Unfortunately going from town to town in close proximity to others while wipping each other's backs into hamburger is not exactly effective social distancing and they ended up spreading the plague more.
Flagellants are something unique for the empire (Bretonnia have something similar called "battle pellegrins", but we will see later the key differences), and the lore behind them is both easy and credible. Life in the Empire is difficult if not shitty. While your position in society surely determines your lifestyle, there aren't assurances that all you have will not be destroyed by one of the many, many enemies of the Empire. Everything that you have will be destroyed or stolen, your family and friends will be killed horribly, taken prisoners or sacrificed to horrid gods who will torture their souls for all eternity. Those who go through such harshness cannot possibly remain sane or hope to retake what they have lost. They will become bandits, desperados and this if they have enough mind to keep their sanity. But for these madmen, faith and vengeance are also an option. Inevitably, a Prophet of doom will arise between this madmen, convinced that what they had seen and gone through are clearly sign of the coming of the apocalypse. The Empire will fall, nothing can save it from such a destiny, and if everyone is doomed, the only "logical" thing to do is to take care of our souls through zealous acts of faith to impress the empire's pantheon, especially Sigmar, the God-king. When this happens, a group of like-minded fanatics will band together with this prophet, and start to travel far and wide the Empire, spreading their message of doom and looking for the only possible outcome, a martyr death in the name of the gods in bloody battlefields. In their bloody pilgrimage, more men will join the band of flagellants, creating a crazy parade of undeniable
madness zealotry. Common people have mixed feelings toward them, a combination of respect, fear and owe. The state church of Sigmar support the flagellants actions, found them to be useful assets. What better retinue for a Witch Hunters or a Warrior priest that devoted people who don't fear death, but actively look for it? Flagellants on the other side obey the official clergymen with awe and fanatical loyalty, because their word is that of Sigmar itself. Some groups of flagellants indeed follow a particularly famous man of the church, such the crazy and obsessed stalkers DEVOTED, LOYAL AND ABSOLUTELY NOT CREEPY FOLLOWERS known as Tattered Souls, who follow Volkmar the Grim wherever he go.
It must be noted, however, that while many flagellants are obvious followers of Sigmar, some band will dedicate themselves to another god of the pantheon, such as Morr, Ulric, Shallya or other. (Someone can actually argue the sense of venerating the goddess of health with flails and self-torture, but flagellants don't give a fuck about logic and common sense.)
Age of Sigmar
And they were right all along... The End Times had come, and with it Age of Sigmar had born... and flagellants get a quality jump of their lore of such awesomess that you will not believe it! first of all, for some time, the flagellants were a stand-alone faction called Devoted of Sigmar, together with warrior priests and the war altar models. with it, they also appear in a series of books who explore their way of life weirdly deep. Flagellants now have many backgrounds: many of them are people from Azyr who are convinced by skilful orator and priests to join the armies of Sigmar and fight to reconquer their ancestor lands, gaining a sure pass towards heavens if they die during the process, turning them into the fantasy versions of the "peasants crusaders". In other cases, they are the ancestors of those unlucky fuckers who didn't reach the Azyr portals in time, and so had travel through the corrupted lands of the realms, until the storm of Sigmar hit the fucker where it hurt, turning these desperados into killer machines, craving vengeance and care little for their own lives. Sometimes they are neither but just low-cost militias or rebels who fight more to free their country and are less into self-torture and that kind of stuff, but the models gave quite the idea of poor militiamen. Until this points this changes will look nothing special to you considering the old lore, but shit truly hit the fan when you read that these fuckers, through processions, rituals of martyrdom and penitence, root out chaos corruption, because where their blood falls, such is their faith that the chaotic influence is burned out, letting people recolonize those lands without fears. All of this is further confirmed by the new battletome Cities of Sigmar, making these guys from those annoying smelly fuckers who roam around the country, shouting nonsense about apocalypse incoming, into one of the most important fuckers of the orders armies, because without them the "reconquista" would be really more difficult. Also, there is a Stormhost who uses them quite frequently, as is depicted in some short stories and cameo, the Tempest Lords (yes, the noblest and prideful of all the Stormhosts has close ties with flagellants, we are reaching shipping tier of bullshit).
Ways to recognize a flagellant
I'm not trying to insult your intelligence with this add on, but the point is that these guys are so damn crazy that their physical description deserve a paragraph in its own right. While with "flagellants" you may think that this guys are simply flailing themselves as a way to purify their souls from their sins, what these guys truly do to themselves is far, far, far worse, making the flailing part look soft in comparison. Seriously, a campaign tome stated that before a battle they didn't eat or sleep, chanting prayers all night long, until morning where they had become weirdly silent, because they cut out their own tongues. The point is that flagellants aren't sane, but straight up crazy. Nails that goes through their limbs, deep daily cuts, chains and weight attached to their bodies, self blindness... you name it! While all of this should make anyone into poor soldiers for the weakness coming from such wounds, FLAGELLANTS DON'T GIVE A FLYING FUCK ABOUT IT, EASILY LIFTING HEAVY WEAPONS LIKE HAMMERS AND FLAILS! They have gone through such terrible stuff that you can't do anything more than what they are already doing to themselves. Simply put, nothing can scare them, NOTHING! This makes them, theoretically, immune to Chaos corruption because the dark gods work around the inner desire of the people they want to corrupt, but the inner desire of a flagellant is death, so logically Chaos can't find a leverage point. This brings us back to their vastly inferior "cousins", the battle pilgrims. Their faith is brought by desperation, because they don't want to continue their life as shitty peasants, making them quite prone to run when shit hit the fan. They also cause turmoil wherever they go, forcing many Bretonnian lords to suppress such groups. Worst of all they have killed many a living grail knight who they had sworn to protect, because after a fall from the horse or an apparently lethal wound, they storm his "corpse", taken by a razing fever that makes Ork lootas clap their hands in admiration. Flagellants are far more useful, don't cause turmoil, are approved and controlled by the official church, and will happily fight that Bloodthirster until their death without being told to do so.
On the tabletop, they are a ludicrously expensive unit that needs to be in numbers of 30+ to even be worth taking, and even then they'll die to anything within a turn or two of combat. Their strength is that they'll take down whatever they're fighting too. For example, say the enemy has a 500pt unit of chaos knights with a fancy chaos lord marching towards your flank; just send in your 300pt unit of flagellants to delete the fuckers. Sure, your flagellants are dead, but so are the enemy, so who cares.
In Age of Sigmar they act as a shocktroop for the Cities of Sigmar. they gain a great attack bonus when they charge, have a good mobility, and gains further attacks when some of their models dies(+2 attacks if 5 models dies, for a maximum of 4 each. not bad at all) and believe me, they will die in droves thanks to the missing of the SAVE ROLLS! still thought, there are many ways for you to implement their durablity thanks to debuff spells and abilities that comes with the collegiate arcane. Also, they will not escape during battleshock, because you roll a dice for every model who had "run", and for each 4+ the closer enemy unit will get a mortal wound. Why? Because they hadn't run, BUT THROW THEMSELVES IN ONE LAST, DESPERATE ATTACK WITH THE HOPE OF KILLING ANOTHER HERETICS WITH THEIR DEATH!
In total Warhammer: War, they are a part of the 'Grim and the Grave' DLC that should have just been in the base fucking game. They are unbreakable, with little to no defence which heavily reduces their survivability compared to even State Troops.