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This achievement can be earned during Mission 7, A Crack in the Slab.

After obtaining the Timepiece from The Outsider, the game will inform you how it can be used: while equipped, tap [ Register or Signin to view external links. ] to alternate between timelines (past/present), and [ Register or Signin to view external links. ] to see a visible indication of what is happening in the other timeline. With this in mind, equip the ancient relic and "time travel" out of the room until you are in the hallway on the first floor. Your destination is on the opposite side the hallway.

Travel back to the present and when you reach the blocked door at the end of the hallway, time travel to the past to get through the blocked door and into the dining room, then return to the present immediately. Move through the room, crawl underneath the dining table, and time travel to the past once more. The room will be filled with guards; ignore the guards, loot the key that is on the chair, and the achievement will unlock.