How to Acquire
To complete this achievement, either Corvo Attano or Emily Kaldwin has to recover the master key of Stilton Manor from the dining room without alerting the guards around, during the mission A Crack in the Slab. To do so, first enter the room in the present, crawl under the table and activate the Timepiece to enter the past. Then, reach and grab the key and use the Timepiece again to go back to the present.
This achievement also unlocks the special action of the same name.