Yasei no Last Boss ga Arawareta!

Lufas Maphaahl rules the land of Mizgarz with an iron fist. Mankind stands no chance of prevailing against the most powerful being in the world and her army of loyal minions—until a band of heroes bests her in an epic battle, sealing her soul away for all eternity.

But this is just another day in Exgate Online. Lufas is a player's avatar and a legend among fans. She even outshines the official last boss, the Devil King. So it's no big deal to the boy behind Lufas when he logs off after his epic defeat—thankfully, it won't really be eternity until he can play again. But he's met by a strange offer, and when he accepts, he's sucked into the game world and awakens...in Lufas's body!

Two hundred years have passed since Lufas's defeat, and Mizgarz now lives under the shadow of the Devil King. How will our hero fix his mess? And can he save the world from the one being he never subordinated: the last boss?

(Source: J-Novel Club)