A eunuch is a man who has been castrated or emasculated, either prior to puberty or afterwards. Most people would not volunteer their young sons to become eunuchs and most men would not choose to be castrated or emasculated. Therefore, most eunuchs are slaves who have had no choice in the matter. The most famous eunuchs are the Unsullied.
Eunuchs in Essos
Eunuchs are far more common in Essos than Westeros, where they make popular slaves. In Yunkai, slaves are made into eunuchs by cutting only the testicles off.  The Yunkai'i also geld boys in order to produce castrati slaves. Yurkhaz zo Yunzak owns a group of Yunkish castrati slaves who sing songs in the ancient tongues of the Old Empire. Their voices are high and sweet and impossibly pure. The Dothraki dosh khaleen are served by eunuchs. In the Seven Kingdoms poison is seen as a weapon of eunuchs.
In Bayasabhad, Kayakayanaya and Shamyriana 99 of 100 boys, the sons of the Great Fathers, are gelded when they reach the age of manhood and live out their lives as eunuchs, serving their cities as scribes, priests, scholars, servants, cooks, farmers, and craftsmen. Only the most promising males, the largest, strongest, and most comely, are permitted to mature, breed, and become Great Fathers in their turn.
The priests of the Blind God, a deity that was once worshiped in the Valyrian Freehold, were eunuchs who wore eyeless hoods in honor of the god, for only in darkness, they believed, would their third eye open.
Eunuchs in Westeros
In Westeros castration appears to serve more as a severe punishment and deterrent.  Varys is a known eunuch who has risen to the influential and powerful position of Master of Whisperers. However, he was made a eunuch in Myr, not Westeros.
- Theon Greyjoy (unconfirmed) Ramsay Bolton may have removed his genitals, as both Ramsay and Theon consistently imply
- The priests of the Blind God
- The Nine Eunuchs - (voluntarily emasculated)
- Varys - (emasculated)
- Grey Worm - (emasculated)
- Hero - (emasculated)
- Stalwart Shield - (emasculated)
- Strong Belwas
- Almost all of the sons of the Great Fathers
- Lysaro Rogare - gelded by all those he had wronged.
- Ser Lucamore Strong – (castrated) - gelded as punishment
- Ser Tyland Lannister – (castrated) - gelded when under torture
- Rodrik Greyjoy - taken to Casterly Rock as a captive where Lady Johanna Lannister had him gelded and made him her son's fool.
A eunuch has no honor, and a spider does not enjoy the luxury of scruples, my lord.
I've never been quite sure what the point of a eunuch is, if truth be told. It seems to me they're only men with the useful bits cut off.—the Queen of Thorns
Even those who lack a man’s parts may still have a man’s heart, Your Grace.
“Have you ever heard such singing , my love?” Hizdahr asked her. “They have the voices of gods, do they not?” “Yes,” she said, “though I wonder if they might not have preferred to have the fruits of men.”
What does a eunuch know of a man's honor?
References and Notes
- A Storm of Swords, Chapter 23, Daenerys II.
- A Dance with Dragons, Chapter 60, The Spurned Suitor.
- A Game of Thrones, Chapter 46, Daenerys V.
- A Game of Thrones, Chapter 25, Eddard V.
- A Dance with Dragons, Chapter 36, Daenerys VI.
- The World of Ice & Fire, The Grasslands.
- A Feast for Crows, Chapter 13, The Soiled Knight
- The World of Ice & Fire, The Red Kraken.
- Fire & Blood, The Lysene Spring and the End of Regency.
- A Game of Thrones, Chapter 58, Eddard XV.
- A Storm of Swords, Chapter 6, Sansa I.
- A Dance with Dragons, Chapter 11, Daenerys II.
- A Dance with Dragons, Chapter 24, The Lost Lord.