Last Round Arthurs, Vol. 4 (light novel)

Now that Rintarou has awakened the power within him, there is something comfortable about his new life...even if he spends his days trying to keep up with Luna's reckless antics. But this current life has come at the cost of Nayuki's. The only way to save her is to get their hands on the Holy Grail, an object that's eluded all search parties since ancient times. As this quest unravels, Rintarou realizes it might be a little more complicated than he imagined...