Kage no Jitsuryokusha ni Naritakute! Master of Garden ~Shichikage Retsuden~

Kage no Jitsuryokusha ni Naritakute! Master of Garden ~Shichikage Retsuden~ summary:

Based on the Gatcha auto battler video game. Focuses on recounting the past of other characters and intermediate stories that occur during the main story which are also canonical.
Not the main character, and not the last boss. The one who lurks in the shadows of the storyline and holds all the power, hiding his true abilities by acting like a side character. The boy who longs to be a 'Shadowbroker' lives an unobtrusive life as a side character, training to gain magic power until he's killed in an accident and reincarnated into another world.
Reborn as Cid Kagenou, the boy decides to enjoy himself in this other world with a made-up scenario about "the Eminence in Shadow." He starts fooling around with secret maneuvers against an evil cult that he born from his imagination... and it turns out that the cult really does exist! The girls he took on as his subordinates on a whim began worshiping Cid as Shadow, and before he knows it, Cid has become a true "Eminence in Shadow." Before long, Cid—along with his mysterious group the Shadow Garden—will destroy the darkness of his new world.
The Strongest MC x Overwhelming Chuunibyou x A Serious Comedy Full of Misunderstandings!