Valis: The Fantasm Soldier, called Mugen Senshi Valis (夢幻戦士ヴァリス) in Japan, is a 1986 platformer by RIOT, a studio at Telenet Japan, for the MSX computer. It was ported to the Sega Mega Drive and released in late 1991. Despite being the first game in the Valis series, the Mega Drive port is a remake that was released after the port of Valis III was.
Yuko is walking when the weather repeatedly changes from raining to dry. Yuko notes that she dreamed about it the night before, and in that dream, she heard a voice call her. Suddenly, her friend Reiko appears, speaking in a very unusual fashion before leaving "for a while." While Yuko wonders why Reiko was acting weird, a monster appears out of nowhere and attacks. The voice from Yuko's dream calls out again — this time coming from someone in real life — and that person throws Yuko the Valis sword, allowing Yuko to attack not only this monster but all the monsters coming in.
That person turns out to be Varia/Valia, a being from fantasy world [sic] (whose name has changed in later translations). fantasy world exists in another dimension, but is bound to the dimension Earth is in ("the real world") by the internal "Yin"/"Yang" (always referred to with quotes) balance of people's souls — which has come out of balance because of Yin's power "Vecanta," which has caused "people" to "entertain" themselves with war. Rogles, a king of another dimension, is behind this manipulation. To ensure his victory, Rogles took "Yang"'s power, put it in the Phantasm Jewel, and broke it into pieces, scattering them across dimensions and guarding them with his best guards. Yuko most now collect the pieces and use "Yang"'s power to defeat Rogles.