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The Broo are the most common of the Chaotic monstrous races from the world of Glorantha.
(Note that much of Glorantha lore is meant to be biased and bent towards the in-universe writer of that particular part of the storyline, so all lore must be taken with a grain of salt as it possibly contains fabrications and mistruths, or some conflicts with other works. Mythology of the world of Glorantha is also intended to represent the advancement of the world, with the gods and heroes of prehistory advancing from intelligent animals to prehuman level intelligence to outright civilization, and developing things like writing or concepts of justice as the peoples of the world do.)
Broo are the mortal children of the goddess Thed, and were originally another race in the world just like any other. Thed was a somewhat powerful goddess that created and ruled them. The recent events of that time were that the God of Justice and resident Odin/Zeus-expy Orlanth had killed the God-Emperor (not that one) Yelm and taken control of the pantheon in order to save creation. Orlanth was new to the whole "justice" thing though, this being a very primitive age of the world before codified written law resulting in Orlanth basically inventing the idea as he went, so when Thed came to him and claimed to have been raped, Orlanth simply allowed her to name her assailant and choose whatever punishment she felt was justice. She named Orlanth's own insane brother Ragnaglar (who had been thrown into something known as the "Sex Pit" during the revolution against Yelm, which had shattered his mind). Orlanth likely would have just dismissed the case as "boys will be boys" had he known his brother was the guilty party before giving his word. Depending on the version of this story Ragnaglar was already her husband and father of the Broo before he committed marital rape against her, or was just referred to thereafter as her "husband" due to the crime and primitive societal ideas with the Broo as the product of the event; either way his rape was brutal and inflicted stabbing wounds on her that never healed which were her evidence of the crime, and the Broo were born into the world originally uncorrupted before the violence against Thed damaged their nature.
Thed chose to become the Goddess of Rape as punishment for the accused and transformed the Broo into a race of rapists to inflict her suffering on the rest of the world, forcing all of creation to confront the hidden shame and openly admit "rape is bad" (remember that this is prehistory, when the monolith is still inspiring monkeys to use tools). This is due to the concept of Justice in the world at the time being a personal affair between victim, criminal, and monarch which was based entirely on revenge rather than making an example to society of the ramifications of criminal behavior coupled with Thed blaming society as a whole rather than just Ragnaglar. Thed's choice as a result begins the concept of justice being something involving a community rather than just an eye for an eye as well as forcing the concept of justice being to prevent wrongdoing rather than just to make both parties suffer equally.
Ragnaglar continued his descent into madness independent of Thed's new Portfolio, and corrupted the very idea of sex which had previously only been a beautiful thing for the creation of offspring (note that despite this, rape still existed as Ragnaglar's was not treated as the first). Basically inventing the idea of fetishism, he had sex with anything he could stick his penis inside from animal to unliving (and dead) things all of which somehow produced offspring; this trait was passed onto the Broo, who now had an extreme of lust and could impregnate everything from men to rocks and dirt.
Thed's own descent into madness took a more controlled route; she formed a triumvirate of evil with Ragnaglar and the Fertility Goddess Mallia (depending on storyteller they either corrupted Mallia willingly or kidnapped and raped/tortured her until she went mad; either way she became the Grim Reaper of the setting, her fertility coming to represent disease and old age as well as all gross and unfortunate deaths). The three insane gods planned to take control of the pantheon and rule the world their own way, and had a three-way which caused Thed to give birth to the literal fucking Devil named Wakboth (who corrupted her from within, turning her legitimately evil rather than Chaotic Neutral) which either created Chaos or gave it an incarnation (once again depending on storyteller). The four divines became the first completely evil beings in the setting. Wakboth killed Ragnaglar so that he would never have siblings to rival him, and coerced/raped his mother to produce servants to lead the Broo legions as he destroys the world.
It should be noted that Wakboth is described as existing prior to this in some accounts, and its suggested that in-universe there was a saying that "Rape is his mother, Madness is his father, Disease was his midwife" and that the expression became literal and applied to gods as it was handed down orally over generations (remember that Glorantha lore is from an in-universe perspective, and after many hundreds of generations the oral mythology warps and merges with other myths). Wakboth is also called Kajabor, although some lore treats the two as different entities with Wakboth killing Kabajor after he devoured forgotten gods.
After the great war between the beings of good and evil, Wakboth was defeated but not before he raped Glorantha itself (AKA Earth). He was killed, sent to hell, and was reborn as Time when the gods decided to stop being crazy assholes and live by a set of rules; Time himself keeps the gods from the mortals, and provides them immortality as long as they don't interfere directly in Glorantha.
Thed was skinned alive (but not killed) by Kyger Litor, the goddess that brought order and rules to the forces of evil, and was then later defeated by Polestar, god of the sky and astronomy as well as some magic, which shattered her. Currently Thed exists all over the world as she was spread to its corners and is kept alive solely by the worship of the Broo. Although still their fucking insane rape goddess, she isn't involved in any evil actions anymore and some lore suggests she has come to represent fertility and some aspects of civilization as most of the various deities of the world become less a bunch of godlike savages and more a group of cultured divine beings.
Broo today are a race of beast-people, marked by the features of various animals (most iconically goats, which they originally resembled in their pre-corrupt state) but also invariably further warped with diseases and Chaos-born mutations, which just adds to their general hideousness. They worship all of their divine parentage (Mallia for the most diseased, Ragnaglar for the most depraved, Wakboth for the most aggressive, and all regardless of other things hold Thed as their beloved god-mother (not godmother)).
Their most infamous power is that they have the ability to "rape anything". No, seriously. As children of Thed, Goddess of Rape, and Ragnaglar, God of Madness and Perversion, Broo have the power to impregnate anything they choose to stick their dick into; the reason they look like many animals instead of just Satyrs is because they usually fuck animals and the resultant Broo-spawn usually more resemble their "mother" and gaining some traits from the father. We don't know if the progenitors need to be female, but since there are accounts of Broo raping and impregnating rocks and trees, they can probably knock up male victims too. Some lore depicts Broo bursting from livestock like Xenomorph Chestbursters, but some lore relies on the mother surviving; it may be case by case or depending on what kind of Broo the father is, or simply another example of in-universe conflicts between sources of lore.
Goats are the "traditional" animals for Broo, but any sort of animal will suffice; sheep, cattle and pig-based Broo are the norm in Dragon Pass, for example. Predators are rarely targeted because they're more likely to kill the Broo as he tries to stick his dick in them. Thed's own Portfolio means Broo don't just rape literally, they rape metaphorically; they rape music with their horrible caterwauling and cacophony of "instruments", rape reason with their insanity, and so on and so forth; in this way all beings and spirits are forced to confront the misery inflicted on her so long ago.
Whether or not there are female Broo seems to depend partially on the sourcebook. Some assert that the Broo are all-male, as that furthers their need to be "children of rape". Others claim that there are female broo, though they tend to favor the Chaos Goddess Malia over Thed, because the latter hates them, and are rare (15% of the population, according to one claim). If there are female Broo, whether or not they can get in on the raping is unknown; fans have speculated that, due to the chaotic nature of the Broo and their spiritual connection to rape, female Broo may either instinctively know the spell that lets a birthing mother "share the pain" with the child's father and/or be able to directly impregnate the father himself, which would certainly explain why they're particularly ill-favored by the male Broo.
If these guys remind you of a particularly Slaaneshi flavor of Beastmen, there's a reason for that; Games Workshop created the Beastmen for Warhammer Fantasy to capitalize on the line of Broo models they had originally produced for RuneQuest, before that contract went tits-up. Ironically, the Beastmen are toned down in comparison to the Broo.
Unlike Beastmen however, Broo are not Always Chaotic Evil. Broo are fully capable of being of the good alignments, they just usually aren't because who the fuck in an ancient world would raise the half-animal monster caused by being raped by a similar unthinking drooling murdermachine. That's the kind of shit that you throw in a cave or build a place to lock the creepy little thing in, assuming you don't just bash its brains out with a club since it was born from your raped wife/mother/daughter/sister/pet dog/brother/tree in the front yard/self. As a cruel Aesop courtesy of Thed that very few beings learn from, rape victims that don't get proper support from society results in more rapists years later. Those that are abandoned are taken in by a Broo clan/horde which will add that hidden shame to their world-destroying mass of throbbing cocks (take that as an animal pun or not). In this way a few Broo can move into an area and increase their numbers by raping trees and deer until eventually they begin forcing themselves on villagers that keep the shame secret and abandon the newborn Broo to die of exposure, which are rescued by the clan which continues growing in number until its too late and they have a horde that destroys the city/civilization until there's nothing left to rape and they disperse in all directions to repeat the cycle elsewhere. Or, y'know, murderhobos come in and kill them all.
BUT: on the off-chance the mother doesn't abandon her horrible offspring, and the community treats it with love and kindness like any other child, Broo become no different than any other being. In fact a well-known Glorantha character called the Wild Healer of the Rockwood Mountains is a kindly old Broo who fulfills the role of Dumbledore/Gandalf/Professor X etc. by acting as a mentor and father-figure to adventurers. Another Broo, Ralzakark, is a Unicorn-headed emperor of the Broo who is a mix of both good and evil who has an organized society far beyond that of most of the rest of the world, as well as a demigod daughter that represents the nature of freedom itself, but Ralzakark is also is vicious in war and has more than a little bit in common with the main character of American Psycho. There are enough non-evil Broo in the Lunar Empire that their army has an entire Broo regiment.
The easiest way for a Broo to become non-evil is Illumination, a process by which an intelligent being realizes that part of what they thought to be reality is fiction (similar to CHIM in The Elder Scrolls) which frees them from the constraints of morality (not to the degree of "Chaotic Stupid" where you can do anything you want, merely that the idea of Cosmos VS Chaos becomes less Good VS Evil and more from a Taoist approach), any of the rules or reprisal of divine beings, and can manipulate magic that is otherwise impossible, allowing them absolute freedom. Since Illumination is heresy outside the Lunar Empire, it is mostly exclusive to Lunar Empire Broo.
Evil Broo live in two states. Feral Broo behave extremely primitive, alone or in small groups and simply increase their numbers by raping things. Wild Broo are Broo that live in tribes with leadership and shamans, and tend to be more likely to raid and take over territory from mankind.