Kaiki Shita Moto Kensei, Shiryouka no Hangyakusha ni Naru

The strongest sword saint, Chrono Gaius, lost everything on that fateful day. His beloved wife and loyal subordinates were all turned into hideous living corpses by his nemesis, Gilgran, and Chrono himself met an unjust death—or so it should have ended. When he next opens his eyes, he finds he has been reborn as the incompetent youngest son of the very man who killed him. Within this weakest of bodies resides a forbidden power that, for brief moments, even surpasses his former prime. Hiding his true identity, playing the role of “family,” and biding his time, he waits for the perfect chance to take revenge.