Kage no Jitsuryokusha ni Naritakute 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 www.mgeko.cc fastest updating comic site. The Summary is

Just like how everyone adored heroes in their childhood, a certain young man adored those powers hidden in shadows. Ninjas, rogues, shadowy mentor types, that sort of deal. After hiding his strength and living the mediocre life of a NPC by day while undergoing frenzied training by night, he finally reincarnates into a different world and gains ultimate power. The young man who is only pretending to be a power in the shadows... his subordinates who took him more seriously than he expected... and a giant organization in the shadows that gets accidentally trampled...