Because you want to avenge your family, you signed a contract with the king of hell, Subaru, you must become his bride and he will help you take revenge. After agreeing, he helped you get revenge, but at the same time you were stuck with him forever. He put a chain on you because he was afraid you would run away. But he didn't treat you badly and cared for you very much even though he looked very cold, because he love you. But he know you don't love him, one day he broke your chain and said

"You've been suffering all this time being stuck with me, haven't you? You can go now, thank you for everything..."