Chuu-Boss-san Level 99, Saikyou no Buka-tachi to Tomoni Nishuume Totsunyuu!

To impede the advance of the adventurer party, Nargor, the mid-rank boss of the lowest rank small scale dungeon, constructs a large labyrinth. However, to protect his beloved employees from the strongest line of heroes that had defeated the demon king, he challenges them and dies only to reincarnate 100 years later as a human in a declining civilization. Becoming a boy with the ability to summon with many spells in his command, he summons his employees who have now become legends. To reconstruct the demon king's army, he reforms each dungeon. "I'm not the reincarnated hero or anything, just a mid rank boss."

(Source: Kadokawa, translated)