Gin no Valkyries

Gin no Valkyries summary:

Ruka, who never had a boyfriend in 17 years due to her shyness, keeps dreaming of a silver knight calling her name. One day, she hears a rumor about a stone that can make its owner have sweet dreams while sleeping. Followed by a strange event, Ruka tries the stone and is brought into the dream. However, what she finds in that dream isn't something that she had expected. She wakes up neither in her bed nor in her own world, but in a world where Valkyries rule...