Manga by author Okano yuu

The protagonist was an eternal copper rank adventurer of frontier area, Lent. He's unlucky as he meet a powerful monster inside of the labyrinth, got defeated, and when he noticed, he already became a bone manSkeleton. Agonizing since he can't enter the town with his current condition, he's recalling about the existence of monster evolution, with a goal to gain a body with muscle, he defeats the monsters of the labyrinth.

The protagonist was an eternal copper rank adventurer of frontier area, Lent. He's unlucky as he meet a powerful monster inside of the labyrinth, got defeated, and when he noticed, he already became a bone manSkeleton. Agonizing since he can't enter the town with his current condition, he's recalling about the existence of monster evolution, with a goal to gain a body with muscle, he defeats the monsters of the labyrinth.

It details the life of a mid-rank soldier who died at the very end of the war against the Demon Race. Fortunately for him, he wakes up to find that he's a baby again, in the embrace of his real mother, years in the past. Determined to avoid the chain of tragedies that took his beloved ones away from him, he takes advantage of the advancements in swordsmanship and magic that developed during the war (as we all know, wars are when many scientific breakthroughs tend to happen), and tries his best to prepare for the demon invasion as well as strengthen himself with maximum efficiency. Description taken from Hot Chocolate Scans.

When the Demon Tribes led by a Demon King appeared and launched a massive war at mankind, all hope seems to be lost, until a brave group of heroes managed to turn the tide and even defeated the Demon King. John Serieux, a mid-ranked soldier of World"s Resistance against the demon"s army, had fortunately been able to witness the Demon King"s fall by the Heroes, but also was unfortunately killed a moment after. Miraculously, he was still able to open his eyes, only to realize that his mind had been sent back to the past when he"s still a baby! With all the memories of his adult years, the knowledge of many scientific breakthroughs during the war, and information of the upcoming great disaster all intact, John is determined to cultivate his potential efficiently and optimally to help change the future for the best.

Resurrected Demon King Synopsis:After a long-standing battle between the demons and humanity, the Demon King Luruslia Nord was laid to rest at the hands of a hero. However, his soul continued to wander. After thousands of years, Luru awakened in a world devoid of demons. Moreover, he was reincarnated as a weak human! To search for his disappeared brethren and to understand the meaning of the new "life" he's been granted, Luru sets forth on a new adventure., Revive the Devil