The Strongest Devil becomes a rookie is a Manga/Manhwa/Manhua in (English/Raw) language, Action series, english chapters have been translated and you can read them here. You are reading chapters on ManhwaZ fastest updating comic site. The Summary is
Amidst the epic clash between the imperial-level boss and the elite players, a legion of monsters perished, leaving heaps of bodies, ultimately vanquished in a final act of sacrificial explosions. However, to everyone's astonishment, the Supreme Boss is reborn seven years prior, but this time, as a clumsy yet charming rookie, with only modest abilities.