Rejected the Prince, Married the Enemy

Reborn on the day Prince Kael proposes, Lirael remembers her painful death after being betrayed and having her magic exploited. She rejects his proposal. To escape, she points to Emperor Hades, lying that they are in love. Hades immediately proposes, confirming their engagement. She realizes he is the enemy who will destroy her country. What scheme is behind this unexpected marriage alliance?