How to Acquire
In Stilton Manor during the mission A Crack in the Slab, Corvo Attano or Emily Kaldwin has to recover a Rune from a walled off part of the basement after draining it. To do so, first enter the past using the Timepiece and grab the crank wheel near the brick wall of the sealed room, then throw it through the small opening. Afterwards, the wheel has to be moved to the other side of the door in this room, but whether this is done in the past (by throwing it through the door) or future (by collecting it from the floor in the past and swimming with it into the other room) does not matter. In either year, the wheel must be attached to the axle and turned to open the vent and drain the water. Recovering the rune from on top of the crate in the future will complete the achievement.
Note that this action can only be done if the present has not been altered from the past, regarding Aramis Stilton's fate. Also note that draining the water in the past means the bloodfly nest is not destroyed, thus is still there in the future. This achievement also constitutes the special action Leaky Basement for the mission's score.