The immortal cup. The provision made for the demons that wants to be with their heartbeat for life. After their marriage, water would be poured inside the immortal cup and given to the heartbeat to drink. She would lose her mortality and her life would be extended to stop death from doing them apart.
That's one of the perks of being loved by a demon as his heartbeat and chosen to be his mate. You get to be made an immortal and be stuck with him forever. Once you drink from that cup, there is no turning back. You lose everything that once made you a human. It is a sacrifice the human has to make if she truly wants to be with her redeemed demon for life.
Why didn't she drink from the cup to prevent her from ever dying and being away from him? Could it be that the love between them was one-sided?
