Don't Make a Girl a Promise
From Halopedia, the Halo wiki
Don't Make a Girl a Promise
Don't Make a Girl a Promise is an achievement in Halo: The Master Chief Collection. It is unlocked by finding and walking over the Cortana toy in the level Gravemind.
The toy only appears in Remastered graphics mode. The achievement is worth 5 gamerpoints and can be unlocked on any difficulty.
Location
The toy is found in the Valley of Tears B. Upon entering the area, proceed to the right and up the rocky face to the metallic platform which curves back toward the wall. The toy is found at the very end of the platform.