A Maiden's Prayer

Otome no Inori

Also known as 乙女の祈り, Slayers Next: A Maiden's Prayer

Searching for a Claire Bible manuscript, Lina and the others find an ancient book describing a musical spell. The spell requires participants to dress up in costumes and chant together. Lina thinks the song being is a holy spell of magical protection but it turns out to be a lost festival song to summon holy light and flower petals. When she discovered this discrepancy, she blows up everyone with a hastily cast Dragon Slave.