Kahwini Musuh Demi Balas Dendam

Her sister marries into another country, but is tortured to death. She swears to avenge her. She enters the palace, and she pretends to be slow and foolish. The emperor likes her, but he also stays wary of her. Others also have their own hidden plans. Will her disguise be seen through? And how will she use the people around her to take revenge?