This effect targets the 'Fortune Lady' monster in your GY.
If a 'Fortune Lady' monster you control battles another monster and both are destroyed, you cannot activate the effect of "Fortune Lady Dark".
If "Fortune Lady Dark" is not face-up when a 'Fortune Lady' monster you control destroys an opponent's monster by battle and sends it to the GY, you cannot activate this effect.
If the 'Fortune Lady' monster that destroys an opponent's monster by battle is not face-up when the monster is sent to the GY, you cannot activate this effect.