Kekkon Yubiwa Monogatari / 結婚指輪物語

A decade ago, princess Krystal Novaty “Hime” Nokanatika traveled from another world to escape assassination attempts by an evil force. When she arrives in modern-day Japan, she encounters Haruto Satou. With him she shares a promise: he must forget what he has just witnessed, and the two will become friends. Now heading back to the Nokanatika Kingdom to fulfill her royal duties, Hime bids farewell to Haruto. However, when the young man suddenly remembers his first meeting with Hime, he rushes to the location where she first appeared and crosses the dimensional portal to be reunited with her. His arrival disrupts the marriage ceremony between Krystal and prince Marmarugias Gisaras. The wedding is brutally interrupted by the attack of an abyss monster who threatens the princess' life. Acting impulsively, Hime decides to exchange rings with Haruto, which grants her new husband light powers that permit him to slay the monster. Now dubbed as the Ring King, Haruto must marry four other princesses to acquire new magical powers and have a chance to prevail in his fight against the enemy of the world: the Abyss King. ...