Department of Mysteries

[Jacob, Dark and most feared wizard of all time, Lord Hott]

The Department of Mysteries, located on Level Nine is a department in the Ministry of Magic which studies particular enigmas (death, time, space, thought, and love) and stores copies of prophecies made in the Harry Potter universe. During Voldemort's discriminatory regime, he forces the department to lie and claim that Muggle-borns actually steal magic from Pure-bloods, making them "illegal magicals" and allowing their arrest.[HP7]
Because of the covert nature surrounding this particular branch of the Wizarding government, the Department of Mysteries can be likened to real-world intelligence agencies like the CIA or MI6, in that most of their operations are kept in total secrecy from the general wizard populace. However, the primary operations of the department seem to be more like those of scientists, the department attempting to uncover the sources and rules that govern magic.

[Jacob, Dark and most feared wizard of all time, Lord Hott]

Unspeakables
The Unspeakables are the group of wizards who work in the Department of Mysteries (their identities classified for security reasons). Known Unspeakables include Broderick Bode, Croaker,[HP4] and Augustus Rookwood who is a Death Eater.