Yamada Fuutarou Manga

Vow of Vengeance. Seventeenth-century Japan: A rebellion in the Aizu teritory has been brutally crushed, leaving twenty one brave warriors dead and most of the nuns of the local convent slaughtered. Now the surviving nuns have sworn to seek revenge. They turn to the greatest swordsman of the land to train them so they can kill the ones who slew the men and the other nuns.