When he was still alive, the unnamed soldier was part of a detachment whose task was to hide a treasure near Flotsam. Eventually the soldier was captured by some Aedirnian scouts, who brought him in a nearby asylum where two medics named Rupert Brandhuber and Gridley interrogated him, learning thus the location of the hidden treasure. After the Nilfgaardian soldier drawed a map representing the treasure's location, he was finally killed. After his death however, the other inmates of the hospital started a revolt and the place was burned to the ground. At last, only a woman among the inmates managed to escape the structure, taking with her even the treasure map.
Years later the wraith of the soldier summoned again Rupert and Gridley in the ruins of the hospital to get revenge, demanding their hearts and their eyes.
If Geralt surrenders Rupert and Gridley to the wraith: In the end Geralt didn't take the defense of the two medics, who were both killed by the Nilfgaardian's wraith.