Agate

Agate is a demon girl cursed with an eternal life. To forget the past, she throws half of her head into a deep valley and runs away. Unexpectedly, the tears flood the valley floor and form into a lake, triggering a flood in hell. Aloys, the Prince of Ghost comes to the Human World to find out the truth and finally finds the demon girl, starting a story of life and love.

(Source: Bilibili YouTube)