The Unsullied are eunuch slave soldiers, trained from a young age in Astapor to unquestioning obedience and martial prowess. They are used as guards all over the Free Cities. They are sold by the century or the thousand. They make excellent guards, do not loot and will never rape. They also worship a goddess of their own, whose name is kept in secret among them but is known as the Lady of Spears, the Bride of Battle and the Mother of Hosts.
They were most famously used in the Battle of Qohor, where three thousand Unsullied held back a Dothraki khalasar over fifty thousand strong. Only 600 of the Unsullied survived, having killed 12,000 of the Dothraki. To honor the Unsullied, the Dothraki rode in before the Unsullied line, throwing their cut braids down in front of them 
Unsullied are all former male slaves chosen young, for their size, speed and strength.
Their training starts a age 5 and is from dawn to dusk. It is brutal, designed not only to teach them how to fight, but to strip away all individuality, empathy, and self-worth. Only one in three survive, and those that fail at any stage are killed.
They are fully castrated - penis and testicles cut - and their manhoods burned at the altar of the Lady of Spears. This means that they cannot be as strong as whole men, but this is more than made up for by discipline. They regularly consume an elixir called the Wine of Courage to deaden their sensitivity to pain. They drink it with every meal, and every year feel less and less pain.
Every day they choose new names at random by drawing tokens from a bucket, each consisting of a colour and a type of vermin, such as "Grey Worm". Slaves that fail any aspect of their training are killed. Only a third of the slaves to enter training survive to become Unsullied.
On the day a boy is cut, he is given a puppy to take care of. At the end of the first year, the boy is made to strangle the puppy. Should he fail to do so, he is killed and fed to the surviving dogs. During training boys are culled whenever they fail a task, be it running all day in full pack, scaling a mountain at night or walking across a bed of coals. To win their spiked cap they must take a silver mark go to the slave markets and buy a newborn slave child and kill it before its mother and pay the slave's owner for his loss.
The slavers used to sell Unsullied in groups of 10 for household guards, but that proved unsound since they mingle with others, and forget who they are. Now they are sold only in groups of a century or thousand. 
The only vice left for the Unsullied is food, so they often grow fat.
They fight in formation as light infantry, equipped with short spears, swords, round shields, and distinctive spiked caps. They fight fearlessly and obey without question. Their elite, highly specialized training makes them most effective in their phalanx formation. They do not ride horses. 
Unsullied uniform are plain and without ornament, quilted tunic, a short sword, three spears and spiked bronze caps. One spike indicates a low rank, while three spikes going from front to back represent an officer.
A Storm of Swords
While becalmed during the voyage to Pentos to join up with Magister Illyrio Mopatis, Ser Jorah convinces Daenerys Targaryen to change their destination to Astapor to buy Unsullied. Dany is initially sceptical since the Unsullied do not ride horses and all the Unsullied she had seen were fat. Ser Jorah tells her she should not judge all Unsullied against some old household guards. He further explains that food is the only vice allowed them. He then tells her about the Battle of Qohor. This would be a great test of Magister Illyrio Mopatis dedication to her cause. To buy the Unsullied they could use the trade goods being carried by the ship.
Dany is shown the Unsullied by the slaver Kraznys mo Nakloz in the Plaza of Pride in Astapor where 10 ranks of 100 Unsullied stand. Their eyes are fixed straight ahead, as they have been for a day and a night. The slaver tells her that they will stand there unmoving until the last had died. This demonstrates their obedience. Dany asks about the castration since un-castrated are said to be stronger. The slaver admits this, but states that they have discipline instead. They fight in fashion of the old empire (like the lock-step legions of old Ghis); they are absolutely obedient, absolutely loyal, and without fear.
After Whitebeard tells the slaver that even brave men show fear when faced with death or maiming, the slaver whips one of the Unsullied across the face, the man does not move. Dany stops him from whipping another. Then the slaver cuts another eunuch, following this he cuts off a nipple. The Unsullied does not move, and when handed back his sword, states he was pleased to serve. Afterwards it is shown they show no fear because of the Wine of Courage.
The slave then tells how in other slave cities a eunuch is made by just removing the testicles but leaving the penis, but only trouble can come of this. The slave then asks Whitebeard about those in Westeros that have sworn to celibacy. Whitebeard confirms this, and the slaver states that there is still temptation, but the Unsullied have no such temptation. Next the slaver tells them that they have no name, only that which they get at the beginning of the day from a name disk taken from a barrel. Whitebeard then exclaims that there is no way a man can remember a new name every day.
Dany is told that an Unsullied graduates by killing a baby in front of its slave mother which is paid to the child’s owner, and that few fail this test. Dany is horrified. Then the slaver reveals that dogs are harder. Each boy is given a puppy when they are cut, and must strangle the puppy at the end of the first year. Any that fail are killed and fed to the other dogs. Next Dany asks whether they can be tempted by freedom. The slave states they cannot be tempted because they have no life outside their duty. Dany then states that it is soldiers she needs. Dany asks how many are available, and is told 8,000, but they are not cheap because their training takes years. They are like Valyrian steel, hammered and folded over and over. They are only trained to fight so officers would have to be provided. Each comes with swords, shield, spear, sandals, quilted tunic, and spiked caps.
Finally, when Dany has no more questions for the slaver, she asks Whitebeard for his opinion in front of the slave girl that is serving as interpreter so the response will get back to the slaver. He says no because there have been no slaves in Westeros for thousands of years; slavery is held to be an abomination, and for this many good men would not support her for this reason. It would also do great dishonor to her house. Dany and Whitebeard continue to discuss where she is to acquire an army to conquer Westeros.
Dany tells the slaver that she will have to have some time to consider. The slaver tells her there is another potential buyer (a Corsair king) that is interested in them all. Dany hears the interpreter (a slave girl) tell the slaver that the Corsair only wanted a hundred (Dany had been pretending she did not understand the slaver’s language so information like this might slip).
When she gets back to the ship, Ser Jorah is waiting for her and tells her that the several slavers had come and gone with many scribes; they had come from the slaver and gone over every inch of the ships. Then when Ser Jorah asks how many men they had to sell, she responds with a slap and angry words that they had no men to sell, but eunuchs made of bricks. She wants to sail immediate from the vile sty that is Astapor, but she needs to by 8,000 brick eunuchs and she must find some way to buy them. Later Ser Jorah follows her, asking to speak to her. She does not really want to talk to him, but agrees. He tells her that she must win Westeros as Aegon the Dragon with steel and dragon-fire, which will mean blood on her hands.
When she talks about the 8,000 Unsullied represent 8,000 puppies and babies, Ser Jorah points out that when King's Landing was sacked there were dead babies, rapes and burning. The Unsullied will not sack, rape or kill without her direction. When Dany asks why the Dothraki have not sacked the city given that the walls are crumbling, and there are no guards, and the Sons of the Harpy that guard the city are weak. Ser Jorah reveals that the Sons of the Harpy are all officers, it is the Unsullied that die, and there are two reasons Astapor has not been attacked: the attackers would know that they would be facing the Unsullied, and the Sons of the Harpy give gifts to those that might attack).
Dany tells the slaver Kraznys mo Nakloz through the slave girl serving as translator that she wants all the Unsullied. There are 8 greatest slavers in front of her. When asked she wants the 6 centuries, and the 8,000 and those in training. The slavers are hesitant to sell those in training since if they fail in battle they will shame them. There is also the issue that they will not have any to sell to the next buyer. Finally Dany is told she can only have the 8,600 if her gold is sufficient. She tells them that she will pay the same for those in training. This provoked an argument among the slavers, some liking having the gold now instead of the future, and others worried about the reputation of the Unsullied if they fail.
The lead slaver tells her that she can have the 8,600, and in a year she can buy another 2,000. She replies that she will be in Westeros in a year. Then she tells them she will pay as much for the boy with its puppy as for a full Unsullied. This does not sway the slavers, and she tells them she will pay double. Then the question of how she will pay; she has enough for a thousand (500 if she were paying double) in good in her ships, and can buy another thousand with her crown. She will not part with her crown because Viserys had sold their mother’s crown. She also tells them that she will not sell any of her people as slaves, but they can have the ships (she had warned the captains that she might have to sell the ships, and they were not happy).
With the ships the slavers agree to 2000. She then offers a dragon despite the bad taste; Whitebeard objects telling her to win her throne with dragons not slaves. She responds to Whitebeard that he should not presume to instruct her and orders Ser Jorah to remove him. The eldest of the slavers agree to sell her all the Unsullied for the black dragon, Drogon (the largest and healthiest), trade goods and the ships. This is agreed with Dany stating “done” and then each of the slavers state “done” and the slave girl states eight times “done.” The slaver tells Dany that the Unsullied will learn her language quickly, but she will need a translator, and so she is given the slave girl, Missandei, who was serving as the translator during the negotiations as a token.
The slave girl Missandei accompanies her when she leaves. After telling Whitebeard not to question her in front of strangers, she asks the slave girl her name in high Valyrian; the slave girl is surprised. Then Dany tells her that she is now free and can leave, but if she decides not to leave, she can serve as one of her handmaidens, but it will be dangerous and uncomfortable. Missandei tells Dany that she will stay.
Dany then asks about the Unsullied. Dany asks Missandei if the Unsullied are fearless and feel no pain and is told they do. Then Dany asks if they are obedient, and Missandei tells her that if she tells them not to breath, they would not. Next Dany asks what to do with the Unsullied after she has won, and the girl tells her that there is good market for blooded Unsullied, but they also make fine guards and excellent watchmen. Then Dany asks how she would know if she sells them that the Unsullied would not be used against her. The slave girl tells her that they would do as commanded without question. Missandei tells Dany that she could command them to fall on their swords. Dany can tell that Missandei would not be happy if she commanded the Unsullied to commit suicide, and gets the slave girl to admit to her that 3 of the Unsullied were her brothers.
Dany brings her whole company with her to the plaza where the Unsullied standing straight and still with the boys in the back standing just as straight and still. Kraznys mo Nakloz tells Missandei that the slaves are here and hers if she can pay. Dany has her people stack the trade good in front of the slaver, telling him there was more that they could not carry that were still on the ships. Kraznys had told her that they are green and that she should blood them early. There are many small city along her route ripe for plunder, and the plunder will her hers alone; the Unsullied have lust for gold. Also that she can send the captives back to Astapor for good prices. Then Dany hands the slaver the end of Drogon’s chain, and then Kraznys hands her a whip with 9 ends, the Harpies’ fingers. When Kraznys announces that it is done, she raised the whip above her head and gallops in front of the Unsullied ranks announcing that it is done.
Drogon does not move from litter no matter how much Kraznys jerked on the chain. Dany returns asking Kraznys if he is having trouble and when he agrees, she said that there is a reason: Dragons are not slaves. Drogon’s fire takes Kraznys in the face. Soon all dragons have been released, and are in the air. The dragons take some of the Astapori soldiers, and arrows from Dany’s retinue take others. When the slavers call for the Unsullied to defend their masters, they do not move. Then Dany commands the Unsullied to slay the Good Masters, the soldiers, and slay every man that wears a Tokar or holds a whip, and to strike the chains off every slave. She then cries out: "Freedom!"
When Dany arrives outside Yunkai, the host standing against them is too great not for her forces to take significant losses. There are also cavalry which she knows the Unsullied could withstand, but not her other troops, who are mostly too weak to fight, and untrained. When she talks with each of the captains of the sell swords she tells each of them that their numbers are too small to stand against her Unsullied. It is the Unsullied under Grey Worm that will attack the Yunkai from right and left while the her horse attack the center. The battle is won with less than a dozen losses in part because the Stormcrows defected to Dany and the Second Sons were drunk.
The Unsullied commander Grey Worm is among those present during planning for the attack on Meereen. Daario Naharis states that the Unsullied could take down the gates with axes since they consider boiling oil to be no more than a warm bath. Grey Worm responds that this is false; although Unsullied would not feel burns like men would, the oil would blind and kill. He then asks that the Unsullied be given a ram so that they would batter down the gate or die. Daenerys Targaryen tells Gray Worm she will not throw away Unsullied lives. Later there is a plan presented to invade the city through the sewers; Daenerys includes the Unsullied as the others she does not believe are appropriate for invading through the sewers. Unsullied are among those that invade Meereen through the sewers.
A Dance with Dragons
Several Unsullied soldiers are murdered by the Sons of the Harpy. When Daenerys Targaryen disappears on Drogons back, her new husband Hizdahr zo Loraq attempts to gain control of the Unsullied but they refuse to obey him. Ser Barristan Selmy however was able to convince them to follow him if the Yunkai should attack Meereen.
Under Daenerys Targaryen
They are the lockstep legions of Old Ghis come again, absolutely obedient, absolutely loyal, and utterly without fear.
That dragon queen’s got the real item, the kind that don’t break and run when you fart in their general direction.
Real life Influence
The Unsullied are most likely fashioned after the Swiss pikemen mercenaries of the late middle ages to early European enlightenment. Swiss pikemen mercenaries reached their zenith of notoriety during the renaissance contemporary to the time of “The War of the Roses” which George R. R. Martin has previously claimed heavily influences his series “A Song of Ice and Fire”. Similar to The Unsullied, Swiss pikemen were sold for hire to most European powers as armies and personal guards however, unlike The Unsullied, were not slaves. Swiss Pikemen revitalized infantry offensive units during a time when heavy cavalry dominated the medieval battle field. This was to so much affect that the Valois Kings of France, in fact, considered it a virtual impossibility to take the field of battle without Swiss pikemen as the infantry core of their armies.
The history of the Swiss Pikemen Mecenary units and The Unsullied are remarkably similar as seen in the near identically of the fictional Battle of Qohor and the real life Battle of St. Jakob an der Birs. The Unsullied rose to prominence due to the battle of Qohor, and similarly the effectiveness of Swiss Pikemen started to gain rapport throughout Europe after the Battle of St. Jakob an der Birs. The Battle of St. Jakob an der Birs consisted of a small force of roughly 1,500 Swiss troops mostly young pikemen against a superior force of 20,000 Armagnac mercenary cavalry hired by France. Immediately the Swiss forces formed three pike squares of five hundred men each, and they fought well when Armagnac cavalry charged again and again and were repulsed. The fighting lasted for several hours and was of an intensity evoking awed commentary from witnesses. In the end the small force of Swiss pikemen where eventually killed to the last man. However this was just the vanguard of a larger army of Swiss nearing 20,000 men. In light of this The Dauphin (later Louis XI of France) formally made peace with the Swiss Confederacy.
In the following years the veracity of the personal Swiss Guards to various families throughout Europe was noted by the head of the Catholic Church. To this day, just as the city guard of Qohor is made up entirely of The Unsullied, the protection of the Vatican City is made up solely of Swiss Guards.
The organization and tactics used by the Unsullied and Swiss pikemen are markedly alike. The pike square was a military tactic developed by the Swiss Confederacy, in response to the encroaching Burgandian Ordonnance army. A pike square generally consisted of about 100 men in a 10×10 formation, which held together tight while extending its pikes out ward to form a brisling offensive formation. The square had great mobility and could move directions quickly, as well as form an offensive unit. Akin to the Unsullied the tactic depended on well trained and drilled troops who could move in unison while in close formation. At the time Swiss Pikemen just like Unsullied were most valued for their extremely high levels of discipline. The 100 man pike square also corresponds to The Unsullied for the fact they are sold specifically by the hundreds.
Also the weapon of choice for The Unsullied, the long spear, is nearly identical to that of the Swiss mercenary units being pikes and halberds.
Additional influence may have derived from the soldiers of antiquity, Greek hoplites and Roman legionaries. The Unsullied are well equipped with spears, shortswords and shields, and are highly disciplined, drilled and trained, just like a Roman legionary. We are also told that the Unsullied build field fortifications or entrenchments while on the march, an attribute which the Roman army also had a passion for.
They fight in either packed phalanxes, or in superior, loose cohort formations ("lockstep formations of the Old Empire"). Roman cohort-formations provided more space for maneuver and were consequently more flexible and responsive. This battle formation allowed the soldiers to take better advantage of their primary weapon, the shortsword (the Greeks relied on spears), and made them less vulnerable to flank attacks. During battle, they used to throw pilums (javelins) before closing in with shortswords and shields.
The Roman army is also well known for its administrative organization, but the organization of the Unsullied seems far less sophisticated in comparison. The buyer is supposed to integrate the troops into their own military organization and doctrine, and has to set his own officers over them. But in each unit (Unsullied are sold by the hundreds or thousands) there are probably several lower-ranked officers or instructors who are sold together with their unit, in order to preserve the integrity of the group.
The Battle of Qohor bears a few similarities to the famous Battle of Thermopylae during the Greco-Persian War.