monyta's manga / #Returner(1)

. . . . 20 y/o orphan died and reincarnated as another world's duke's son, where he trained hard to become a holy knight. Over his time he gained power and glory, however his world was doomed. Demons of all sorts kept spawning and destroying everything, killing all life there was except for him. He was the sole survivor, who for 300 years managed to kill every single demon that had appeared, but there was nothing left of his world. On his last kill spree a gate appeared and some earthlings emerged from it - appearantly some time after his first life death earth was "bombarded" with gates and monsters coming out of them, and his gate (his world) so far was the worst out of all, but king cleared it all on his own. Now he lives on earth once again, as a king tho. . . . . . .