Quis custodiet ipsos custodes?
From Halopedia, the Halo wiki
Quis custodiet ipsos custodes? is an achievement in Halo: The Master Chief Collection. It is awarded for activating the seventh Halo: Combat Evolved Anniversary Terminal on The Library. The achievement is worth 5 gamerpoints and can be unlocked on any difficulty.
Location
The seventh terminal is located on the Library's third floor in the large circular room with the Index platform above it. Go either right or left and jump over the low barriers to the opposite side. The terminal flashes yellow, and can be clearly seen across the chasm.
Trivia
The name of the achievement is Latin, and translates to "Who will watch the watchers?" The phrase is from the Satires of the Roman poet Juvenal (Satire VI, lines 347–8), and is typically posed in regard to placing checks on those in positions of power, in this case, an increasingly rampant 343 Guilty Spark. The terminal's description, "Who monitors the Monitor?" is also a play on the phrase.
