Kage no Jitsuryokusha ni Naritakute

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...
This is the story of a young boy who had adored powers in shadows possibly eventually reigning over the world of shadows in another world.