Shizuki Byashiko: Facing your idol

Shizuki Byashiko is a girl who, since childhood, was part of a Mahou Shoujo duo called "Ruby and Saphire". One day, during the attack of beings from the underworld, Ruby was captured and Saphire was killed. On that day, mourning was declared for the two heroines. Their families were extremely affected by the loss of both. Years passed and Shizuki ended up gaining powers and became "the Mahou Shoujo Knight Excalibur". Her wish is to honor Ruby and Saphire. However, one day, the beings from the underworld returned, and the one who freed them was Ruby, now completely corrupted and wanting to destroy the world...