From Halopedia, the Halo wiki
Sacrifice is an achievement in Halo: The Master Chief Collection. It is awarded for activating the second Halo: Combat Evolved Anniversary Terminal on Halo. The achievement is worth 5 gamerpoints and can be unlocked on any difficulty.
Near the start of the tunnel, there is a ledge leading to the next section. Continuing on the route rather than making the jump will lead the player past a small alcove. The second terminal is located at the far end of this alcove.