Who were eunuchs? What role did they play in Persian society?

The sad truth is that human rights in the ancient world depended on socio-economic status, ethnicity, gender and sheer good luck. Take the example of eunuchs in Achaemenid Persian society. Young men were forcibly castrated as part of the slave trade and spent their lives serving their owners. A rare few were owned by the royal family and rose to positions of prominence at the royal court. These were involved in court intrigues throughout the Achaemenid period. During the later Achaemenid times some plotted the rise and fall of their masters, including the kings. This source-based homework task is designed for NSW Higher School Certificate Ancient History students:
Higher School Certificate course
Historical Periods
Option D: Persia - Cyrus II to the death of Darius III
Historical Societies
Option D: Persian society in the time of Darius and Xerxes
Historical Personalities
Option D: The Near East - Xerxes