Vengeance in Darkness volume one: Shadows of Betrayal

Summoned to the magical kingdom of Theadore alongside his top students, brilliant immortal scientist Hayashi Akira is exploited for his craft, betrayed, and left for dead by those he trusted. Refusing to perish, Akira unlocks latent dark powers, accepts an offer from the Demon Lord to become "Dark"—the Third Dark Lord—and builds the multi-racial Devil Army to exact vengeance on the corrupt monarchy. After training his former students to survive the coming storm and surviving a catastrophic war that leaves his forces scattered, Akira disguises himself as Mayor Darastus Kizen to found the hidden sanctuary of BlackBlood. Five years later, his former student Natsumi escapes captivity and inadvertently uncovers the underground base where the remnant Dark Army secretly trains, setting the stage for the legendary leader's eventual reemergence from the shadows.