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After accepting ShadowChild's figurine request, you will receive a new objective that asks you plant the object in a specific office inside the Blade. The office in question is located on Level 4 on left-hand side of the chamber. Your HUD/map should have this office marked, too.

Once you're in the office, simply interact with the desk to plant the figurine and the achievement will unlock. Make sure to access the laptop in this office and read all of the emails in order to gain progress towards "Murder He Wrote."