The World's Least Interesting Master Swordsman: Volume 6

"I've spent three thousand years preparing to kill you--" Fukei has plunged the Kingdom into darkness using storm clouds under his command. An Immortal who has spent millennia training to defeat Suiboku, Fukei commands the heavens and earth and is nigh unkillable due to his Immortal Arts. Even wielding the power of the Legendary Sword Eckesachs, Saiga can do little against this overwhelming force. As Fukei threatens to destroy the Arcana Kingdom, Suiboku returns to the mortal world after thousands of years to save his apprentice's students and to apologize to Fukei. Apprentices who trained as brothers now face off, marshaling centuries of skill and power in a tragic and bitter fight to the death.

[From J-Novel Club]