Court Eunuchs
The castrated palace servants who became, in China and the Ottoman Empire alike, the men who actually controlled access to the throne.
Eunuchs appear at the center of power in China, Persia, Byzantium, the Ottoman Empire, and half a dozen other monarchies, and they are almost always described in the sources that survive as scheming, corrupt, and responsible for whatever went wrong. That consensus should be treated with suspicion, because those sources were written by the scholar-officials and aristocrats the eunuchs were competing with. What is beyond dispute is the structural logic that put them there, which is one of the most elegant and most disastrous pieces of institutional design in premodern government.
The logic
A hereditary monarch has one problem above all others: everyone near him wants something for their family. Ministers build dynasties, generals marry into the aristocracy, and in-laws use access to the throne to enrich their clan. Trusted servants become the founders of rival houses.
A eunuch cannot found a house. He has no legitimate heirs, no prospect of a dynasty, and no place in a kinship network that could rival the ruling one. His entire standing depends on one person. In an environment where every other adviser has divided loyalties, that is an enormous qualification, and rulers noticed it independently across Eurasia.
He also had a second qualification that mattered specifically. A palace containing the ruler's women needed male staff who could move through it without threatening the succession. So eunuchs got the two most valuable things in any court: physical proximity and control of the door. Everything else follows from controlling who reaches the emperor and what he is told.
China
The Chinese case is the most consequential. Eunuchs served the imperial household from the earliest dynasties, and the tension between them and the Confucian scholar-officials produced by the examination system is one of the permanent structures of Chinese politics.
They were not merely servants. Zheng He, the admiral who took enormous fleets across the Indian Ocean as far as East Africa in the early fifteenth century, was a eunuch, captured as a boy in Yunnan and castrated. The Ming emperors used eunuchs as a counterweight to a bureaucracy they did not trust, giving them control of a secret police apparatus, the palace memorials, and eventually the tax and military supply systems. The eunuch establishment grew to many thousands of people, an entire parallel government inside the Forbidden City.
The most notorious, Wei Zhongxian, effectively ruled China in the 1620s while the emperor devoted himself to carpentry, and had temples built to himself while purging his critics. The scholar-officials never forgave the institution, and the histories they wrote made eunuch influence the standard explanation for dynastic decline. There is truth in it and there is also a factional axe being ground on every page.
The Qing sharply reduced eunuch numbers and power after seeing what the Ming had done. The last of them were still living in the Forbidden City when Puyi was expelled in 1924.
The Ottomans and the end
The Ottoman palace used two separate eunuch corps, and their standing tells you a lot about the empire. The white eunuchs, mostly from the Caucasus and the Balkans, ran the palace school and the outer household. The black eunuchs, brought largely from the upper Nile and Ethiopia via the slave trade, guarded the harem, and their chief, the Kizlar Agha, became one of the most powerful officials in the empire, controlling the endowments of Mecca and Medina and holding a rank comparable to the grand vizier. In the period sometimes called the sultanate of women, when royal mothers exercised enormous influence, the Chief Black Eunuch was the channel through which that influence reached the state.
The institution died with the monarchies that housed it, and its ugly foundation deserves stating plainly: it depended on the mutilation of children, usually enslaved and usually foreign, with a mortality rate from the operation that was appalling.
What is worth carrying forward is the design problem, which did not die at all. Every ruler still needs advisers whose interests cannot diverge from his, and still ends up trusting whoever controls the schedule and the paperwork over whoever holds the grander title. Palace chiefs of staff, private secretaries, and personal aides in modern governments accumulate power for exactly the reason the eunuchs did: they are close, they are dependent, and they decide who gets in the room.