Clear and Present Danger
From Halopedia, the Halo wiki
Clear and Present Danger
Clear and Present Danger is an achievement in Halo: The Master Chief Collection. It is awarded by killing the cloaked SpecOps Elite at the start of Winter Contingency on Legendary difficulty in Halo: Reach.
Trivia
"Clear and present danger" is an archaic United States legal doctrine and the namesake of a Tom Clancy novel and its film adaptation. Here, it is a reference to the fact that the Covenant is on Reach, which should be self-evident given the Sangheili that was killed for this achievement. Interestingly, Noble Team does not react upon seeing the dead Sangheili.