Ys IV: Mask of the Sun is the first of two Ys titles released under the heading "Ys IV", the other being Ys IV: The Dawn of Ys. Instead of series developer Falcom creating this game, they licensed it out to Tonkin House, who also published it, and released it exclusively for the Super Famicom on November 19, 1993. A Video Game Remake headed by Arc System Works and published by Taito for the Sony PlayStation 2 called Ys IV: Mask of the Sun - A New Theory was released on May 26, 2005.
The first Prequel of the series (something future titles would also use as a narrative device), roughly two years after his adventures on the island of Esteria, Adol Christin comes across a bottle containing a letter written in a foreign language. When Adol asks his friend Luta Gemma to translate it, the letter reveals it's an SOS from the region of Celceta on the Eurasia mainland, encouraging Adol to head there to investigate the circumstances behind it.
Returning to the birds-eye-view perspective and "Bump Attack" combat system, Mask of the Sun eschews the change in formula of Ys III: Wanderers of Ys in favor of the familiarity from Ys: Ancient Ys Vanished ~ Omen and Ys II: Ancient Ys Vanished The Final Chapter. In regards to magic, Adol no longer uses rings or wands, but rather elemental swords to cast different types of offensive spells. This game also marks the debut of "The Romun Empire", a major antagonist who would reappear in the Sequels.
When Falcom released an official Ys timeline, Mask of the Sun was chosen as the canon version of Ys IV instead of The Dawn of Ys, as it stayed closer to the Falcom's vision. Both games would ultimately be Retconned out by Video Game Remake Ys: Memories of Celceta, which integrates elements from both Mask of the Sun and The Dawn of Ys.
This work provides the following examples:
- Canon Discontinuity: It was previously the canon version of Ys IV until Memories of Celceta replaced it in the official timeline.
- Chekhov's Gun: The key to raising the Ancient City is the Gold Pedestal you sold to Pim in the first game.
- Grail in the Garbage: The Gold Pedestal that Adol sold to Pim's Trading Shop in the first game is the key to raise the Ancient City.
- Hopeless Boss Fight: The first and third phases of the Final Boss in Mask of the Sun. After dodging Eldeel's lightning attacks for a minute, Adol will solve the Moon Logic Puzzle and figure that you have to use the MacGuffin necklace on him. During the third phase, after dodging him again for a while, the Disciples will destroy his armor with a group attack.
- Infinity +1 Sword: The Violent Light Sword and armor in Mask of the Sun, again useless against the Big Bad Eldeel.
- Point of No Return: The Flame Cave in Mask Of The Sun gets destroyed by the raising of the ancient city when it happens.