This is the special key that opens the lock to Diva's cell iIn the tower. It is made of gold and is highly ornated with an intricate design. It is kept under Joel's custody with his other valuable items. Other than Joel, only Amshel has access to the key.
In the Anime
Saya keeps looking for the key after she promises to free Diva. She finds it out and manages to procure it when Joel is absent from his study. She uses it on his 72nd birthday to open the lock of the cell and frees Diva to let her come out and sing. But that incident starts the feud between the sisters.
Although Saya looks for the key, she doesn't find it until Amshel 'accidentally' leaves it behind. Actually, it was planned by Amshel to let Saya free Diva rather than himself.