The Ys Anime: Two OVA Series
Eleven episodes: the whole duology, animated while the games were still new.
Ys reached anime while the ink was barely dry. The first OVA series, Ys: Ancient Ys Vanished, ran seven episodes from November 1989 to September 1991, adapting the first game and expanding its thin plot, including the prologue that players previously knew only from the Japanese manual. Adol is voiced by Takeshi Kusao; the score leans on Falcom's own music, lent by Sound Team jdk.
A four-episode sequel, Ys II: Castle in the Heavens (1992 to 1993), loosely adapts the second game under its own subtitle to avoid the Book I/Book II confusion. Plans for further series covering Felghana and Celceta were discussed and never made. Both series reached English DVD from Media Blasters, and they remain the only screen Ys there is.
WHAT THE ANIME ADDS
Canon by expansionSlaff starts here
The character Slaff first appeared in this anime, then walked backwards into the games in Ys Eternal: a rare case of an adaptation authoring the source.
Promarock
The anime's early town of Promarock later appeared in Ys IV, another thread the animators contributed to the series' fabric.
The sound of the games
Because Falcom scored the series itself, the OVA is one of the few game adaptations of its era that sounds exactly like its source feels.