Fuyajou no Assassin

Pretending to be a little girl whose father was kidnaped, she and a group of Chaldeans penetrate the palace of Dahut. While Dahut is distracted by the battle, she steals the key to the floodgates of the city. Assassin gloats to both parties and leave. And Ys, like in its legend, is submerged, leaving little for the Resistance to save or salvage. (Source: TYPE-MOON Wiki)