The Lost Soul is a mysterious character appearing in Castlevania: Lords of Shadow - Mirror of Fate. This character represents the fate of all four characters: Simon, Alucard, Trevor and Gabriel Belmont.
The Lost Soul chronologically first appears to Trevor at the Entrance Bridge. It informs Trevor that he is unable to pass into the Castle Hall for the moment. Later, when Trevor enters the Castle Hall, the Lost Soul welcomes Trevor, and promises to guide him in order to ensure that everything goes as fate has determined it. Trevor refuses its guidance, but the soul warns him that his journey will be ill-fated and his wife and son are in danger. Wanting to hear no more, Trevor attacks the Lost Soul, shattering part of its face and rendering it mute. The Lost Soul fades away after this.
The Lost Soul appears at various points, watching Trevor's progress and giving him the piece of a door necessary to continue through the castle.
Later, when Alucard awakens, the Lost Soul watches him. Simon also encounters the Lost Soul, which shows him his father's Combat Cross through the Mirror of Fate. The Lost Soul promises (through nods and shakes) not to stand in Simon's way, and to assist him. It then fades away. The soul appears at various points commonly through Simon's journey, though it does not interact with him.
After Gabriel's defeat at the hands of Alucard and Simon, the Lost Soul gestures to Simon, wishing for the piece of the Mirror of Fate that he carried. At first Simon refuses, believing that it belongs to him, but Alucard convinces him to give it to the soul. The Lost Soul then fades away once more. Although it is never hinted at or suggested in the game story itself, according to the bestiary entry for the Lost Soul, which is unlocked upon beating the game, the Lost Soul is an incarnation of fate itself. The Lost Soul has never been seen since the Mirror of Fate's destruction, which may suggest it needed the mirror in order to physically manifest in the world.