The Moon of Ortona

Trivia is a princess of the Moon Kingdom who was under house arrest in the castle. One night she sneaked out during a festival and met a boy with mysterious eyes. She promised him that she would travel the world with him when she grew up. And with that promise, Trivia returned to her normal, cramped life.

Eight years later, she was planning to leave for her journey after her "coming of age ceremony" the next day, to fulfill the promise she made to that boy long ago. However, that was when tragedy struck. This is a fantastical story of a princess that begins with a single "promise."

(Source: Kadokawa, translated)