The Torturer's Quaternionic Groan is one of Anton Sokolov's paintings and a collectible in both Dishonored and Dishonored 2.
In Dishonored 2
During: A Crack in the Slab
This painting is found in the lab area near the Butterfly room on the second floor of Aramis Stilton's Manor.
In Dishonored
During: The Flooded District
This painting is found in the Rudshore Chamber of Commerce building, where Daud is headquartered. It is one of two Sokolov paintings in the Flooded District.